<PAGE>

        As filed with the Securities and Exchange Commission on October 18, 2000
                                                      Registration No. 333-_____

================================================================================

                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549
                                 _____________

                                   FORM S-8

                         REGISTRATION STATEMENT UNDER
                          THE SECURITIES ACT OF 1933

                              AVICI SYSTEMS INC.
            (Exact Name of Registrant as specified in its charter)

               Delaware                                02-0493372
   (State or other jurisdiction of        (I.R.S. Employer Identification No.)
    incorporation or organization)

                             101 Billerica Avenue
                           North Billerica, MA 01862
                                (978) 964-2000
              (Address of Principal Executive Offices) (Zip Code)

                          __________________________

                              AVICI SYSTEMS INC.
                           1997 Stock Incentive Plan
                     2000 Stock Option and Incentive Plan
                 2000 Non-Employee Director Stock Option Plan
                       2000 Employee Stock Purchase Plan
                           (Full title of the plans)

                          __________________________

                            Peter C. Anastos, Esq.
                              Avici Systems Inc.
                             101 Billerica Avenue
                           North Billerica, MA 01862
              (Name and Address of Agent for Service of Process)

                                (978) 964-2000
         (Telephone Number, Including Area Code, of Agent For Service)

                          __________________________

                                   Copy to:

                             John A. Meltaus, Esq.
                        Testa, Hurwitz & Thibeault, LLP
                                125 High Street
                          Boston, Massachusetts 02110
                                (617) 248-7000

================================================================================


================================================================================

                        CALCULATION OF REGISTRATION FEE

================================================================================


<TABLE>
<CAPTION>
                                                                   Proposed
                                                                   Maximum        Proposed
                                                                   Offering       Maximum
Title of Securities                                 Amount to be  Price Per      Aggregate         Amount of
to be Registered                                     Registered     Share      Offering Price    Registration
                                                     ----------     -----      --------------    ------------
                                                                                                    Fee(1)
                                                                                                    ------
<S>                                                 <C>            <C>        <C>                 <C>

1997 Stock Incentive Plan
Common Stock (par value $.0001 per share)             150,000(2)   $  0.01    $      1,500.00     $      0.40
                                                       97,125(2)   $  0.10    $      9,712.50     $      2.56
                                                      198,999(2)   $  0.50    $     99,499.50     $     26.27
                                                      464,894(2)   $  1.00    $    464,894.00     $    122.73
                                                      542,179(2)   $  2.00    $  1,084,358.00     $    286.27
                                                      220,669(2)   $  3.00    $    662,007.00     $    174.77
                                                    3,455,604(2)   $  4.00    $ 13,822,416.00     $  3,649.12
                                                      887,500(2)   $ 12.00    $ 10,650,000.00     $  2,811.60
                                                      867,200(2)   $ 15.00    $ 13,008,000.00     $  3,434.11
                                                      440,000(2)   $ 25.00    $ 11,000,000.00     $  2,904.00
                                                      400,000(2)   $ 31.00    $ 12,400,000.00     $  3,273.60
                                                      200,000(2)   $ 96.75    $ 19,350,000.00     $  5,108.40

2000 Stock Option and Incentive Plan
Common Stock (par value $.0001 per share)             201,000(2)   $ 88.38    $ 17,764,380.00     $  4,689.80
                                                      130,000(2)   $102.00    $ 13,260,000.00     $  3,500.64
                                                       42,500(2)   $115.98    $  4,929,150.00     $  1,301.30
                                                       14,000(2)   $116.06    $  1,624,840.00     $    428.96
                                                          700(2)   $133.00    $     93,100.00     $     24.58
                                                    2,770,593(3)   $ 83.19    $230,485,631.67     $ 60,848.21

2000 Non-Employee Director Stock
Option Plan
Common Stock (par value $.0001 per share)             200,000(2)   $ 31.00    $  6,200,000.00     $  1,636.80
                                                      200,000(3)   $ 83.19    $ 16,638,000.00     $  4,392.43



2000 Employee Stock Purchase Plan                     750,000(3)   $ 83.19    $ 62,392,500.00     $ 16,471.62
Common Stock (par value $.0001 per share)

Total                                              12,232,963                 $435,939,988.67     $115,088.16
-------------------------------------------------------------------------------------------------------------
</TABLE>

(1)  Calculated pursuant to Section 6(b) of the Securities Act of 1933, as
     amended.

(2)  Such shares are issuable upon the exercise of outstanding options with
     fixed exercise prices. Pursuant to Rule 457(h), the aggregate offering
     price and the fee have been computed upon the basis of the price at which
     the options may be exercised. The offering price per share set forth for
     such shares is the exercise price per share at which such options are
     exercisable.

(3)  The price of $83.19 per share, which is the average of the high and low
     price of the Common Stock of the Registrant as reported on the Nasdaq
     National Market on October 17, 2000, is set forth solely for purposes of
     calculating the filing fee pursuant to Rule 457(c) of the Securities Act
     and has been used only for those shares without a fixed exercise price.
<PAGE>

                                    PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

Item 1. Plan Information.
        ----------------

        The documents containing the information specified in this Item 1 will
be sent or given to employees, directors or others as specified by Rule
428(b)(1). In accordance with the rules and regulations of the Securities and
Exchange Commission (the "Commission") and the instructions to Form S-8, such
documents are not being filed with the Commission either as part of this
Registration Statement or as prospectuses or prospectus supplements pursuant to
Rule 424.

Item 2. Registrant Information and Employee Plan Annual Information.
        -----------------------------------------------------------

        The documents containing the information specified in this Item 2 will
be sent or given to employees, directors or others as specified by Rule 428(b).
In accordance with the rules and regulations of the Commission and the
instructions to Form S-8, such documents are not being filed with the Commission
either as part of this Registration Statement or as prospectuses or prospectus
supplements pursuant to Rule 424.


                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.
        ---------------------------------------

        The following documents filed with the Commission are incorporated by
reference in this Registration Statement:

(a)     Registrant's Prospectus dated July 27, 2000, as filed with the
        Commission pursuant to Rule 424(b)(1) of the Securities Act of 1933, as
        amended (the "Securities Act of 1933"), on July 27, 2000;

(b)     The section entitled "Description of Registrant's Securities to be
        Registered" contained in the Registrant's Registration Statement on Form
        8-A (File No. 000-30865) filed on June 21, 2000 pursuant to Section
        12(g) of the Securities Exchange Act of 1934, as amended (the "Exchange
        Act").

(c)     The Registrant's Quarterly Report on Form 10-Q for the fiscal quarter
        ended June 30, 2000 filed on August 31, 2000 pursuant to the Exchange
        Act.

        All documents subsequently filed with the Commission by the Registrant
pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to
the filing of a post-effective amendment which indicates that all securities
offered herein have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this Registration
Statement and to be a part thereof from the date of filing of such documents.

Item 4. Description of Securities.
        -------------------------

        Not applicable.
<PAGE>

Item 5. Interest of Named Experts and Counsel.
        -------------------------------------

        Not applicable.

Item 6. Indemnification of Directors and Officers.
        -----------------------------------------

        The Delaware General Corporation Law and the Registrant's charter
provide for indemnification of the Registrant's directors and officers for
liabilities and expenses that they may incur in such capacities. In general,
directors and officers are indemnified with respect to actions taken in good
faith in a manner reasonably believed to be in, or not opposed to, the best
interests of the Registrant, and with respect to any criminal action or
proceeding, actions that the indemnitee had no reasonable cause to believe were
unlawful.

        The underwriting agreement between the Registrant and the underwriters
listed therein provides that the underwriters are obligated, under certain
circumstances, to indemnify directors, officers and controlling persons of the
Registrant against certain liabilities, including liabilities under the
Securities Act of 1933. Reference is made to the form of underwriting agreement
filed as Exhibit 1.1 to the Registrant's Registration Statement on Form S-1
(File No. 333-37316).

        The Registrant has in effect a directors' and officers' insurance
policy.

Item 7. Exemption From Registration Claimed.
        -----------------------------------

        Not applicable.

Item 8. Exhibits.
        --------

   Exhibit No.  Description
   -----------  -----------

   4.1          Specimen Certificate for shares of the Registrant's Common Stock
                (filed as Exhibit 4.1 to the Registrant's Registration Statement
                on Form S-1 (File No. 333-37316) and incorporated herein by
                reference)

   4.2          Fourth Restated Certificate of Incorporation of the Registrant
                (filed as Exhibit 3.2 to the Registration Statement on Form S-1
                (File No. 333-37316) and incorporated herein by reference)

   4.3          Amended and Restated By-Laws of the Registrant (filed as Exhibit
                3.4 to the Registration Statement on Form S-1 (File No. 333-
                37316) and incorporated herein by reference)

   4.4          1997 Stock Incentive Plan (filed as Exhibit 10.1 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.5          2000 Stock Option and Incentive Plan (filed as Exhibit 10.2 to
                the Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.6          2000 Non-Employee Director Plan (filed as Exhibit 10.4 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by
<PAGE>

                reference)

   4.7          2000 Employee Stock Purchase Plan (filed as Exhibit 10.3 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.8          Fifth Amended and Restated Investor Rights Agreement, by and
                among the registrant and the Investors and Founders listed
                therein, dated as of April 24, 2000, as amended by Amendment No.
                1 dated July 7, 2000 (filed as Exhibit 10.5 to the Registration
                Statement on Form S-1 (File No. 333-37316) and incorporated
                herein by reference)

   5.1          Opinion of Testa, Hurwitz & Thibeault, LLP

   23.1         Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit
                5.1)

   23.2         Consent of Arthur Andersen LLP

   24.1         Power of Attorney (included as part of the signature page of
                this Registration Statement)

Item 9. Undertakings.
        ------------

        The undersigned Registrant hereby undertakes to file, during any period
in which offers or sales are being made, a post-effective amendment to this
registration statement:

          (i)    to include any prospectus required by Section 10(a)(3) of the
        Securities Act of 1933;

          (ii)   to reflect in the prospectus any facts or events arising after
        the effective date of the registration statement (or the most recent
        post-effective amendment thereof) which, individually or in the
        aggregate, represent a fundamental change in the information set forth
        in the registration statement; and

          (iii)  to include any material information with respect to the plan of
        distribution not previously disclosed in the registration statement or
        any material change to such information in the registration statement;

        provided, however, that paragraphs (i) and (ii) above do not apply if
        the Registration Statement is on Form S-3, Form S-8 or Form F-3, and the
        information required to be included in a post-effective amendment by
        those paragraphs is contained in periodic reports filed with or
        furnished to the Commission by the Registrant pursuant to Section 13 or
        15(d) of the Exchange Act that are incorporated by reference in the
        registration statement.

        The undersigned Registrant hereby undertakes (i) that, for the purpose
of determining any liability under the Securities Act of 1933, each such post-
effective amendment shall be deemed to be a new registration statement relating
to the securities offered therein, and the offering of such securities at that
time shall be deemed to be the initial bona fide offering thereof; and (ii) to
remove from registration by means of a post-effective amendment any of the
securities being registered which remain unsold at the termination of the
offering.
<PAGE>

     The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or 15(d) of the Exchange
Act that is incorporated by reference herein shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
Registrant pursuant to the foregoing provisions, or otherwise, the Registrant
has been advised that in the opinion of the Commission such indemnification is
against public policy as expressed in the Securities Act of 1933 and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the Registrant of expenses incurred
or paid by a director, officer or controlling person of the Registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act of 1933 and will be governed by the
final adjudication of such issue.
<PAGE>

                                  SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Billerica in the State of Massachusetts, on this 18th
day of October, 2000.

                                    AVICI SYSTEMS INC.


                                    By:  /s/ Paul F. Brauneis
                                         --------------------
                                         Paul F. Brauneis
                                         Chief Financial Officer and Treasurer

                       POWER OF ATTORNEY AND SIGNATURES

     We, the undersigned officers and directors of Avici Systems Inc., hereby
severally constitute and appoint Paul F. Brauneis, our true and lawful attorney,
with full power to him, to sign for us in our names in the capacities indicated
below, any amendments to this Registration Statement on Form S-8 (including
post-effective amendments), and to file the same, with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, and generally to do all things in our names and on our behalf in our
capacities as officers and directors to enable Avici Systems Inc., to comply
with the provisions of the Securities Act of 1933, as amended, hereby ratifying
and confirming our signatures as they may be signed by our said attorneys, or
any of them, to said Registration Statement and all amendments thereto.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.


             SIGNATURE                                         TITLE

/s/ Surya R. Panditi                  President, Chief Executive Officer
---------------------------           and Director (principal executive officer)
Surya R. Panditi

/s/ Steven B. Kaufman                 Director and Chief Operating Officer
---------------------------
Steven B. Kaufman

/s/ Paul F. Brauneis                  Chief Financial Officer, Treasurer and
---------------------------           Secretary (principal financial officer and
Paul F. Brauneis                      principal accounting officer)

/s James R. Swartz                    Chairman and Director
---------------------------
James R. Swartz

/s/ Stephen M. Diamond                Director
---------------------------
Stephen M. Diamond

/s/ Catherine M. Hapka                Director
---------------------------
Catherine M. Hapka

/s/ Richard T. Liebhaber              Director
---------------------------
Richard T. Liebhaber

/s/ James Mongiello                   Director
---------------------------
James Mongiello

/s/ Henry Zannini                     Director
---------------------------
Henry Zannini
<PAGE>

                               INDEX TO EXHIBITS


Exhibit No.     Description
-----------     -----------

4.1             Specimen Certificate for shares of the Registrant's Common Stock
                (filed as Exhibit 4.1 to the Registrant's Registration Statement
                on Form S-1 (File No. 333-37316) and incorporated herein by
                reference)

4.2             Fourth Restated Certificate of Incorporation of the Registrant
                (filed as Exhibit 3.2 to the Registration Statement on Form S-1
                (File No. 333-37316) and incorporated herein by reference)

4.3             Amended and Restated By-Laws of the Registrant (filed as Exhibit
                3.4 to the Registration Statement on Form S-1 (File No. 333-
                37316) and incorporated herein by reference)

4.4             1997 Stock Incentive Plan (filed as Exhibit 10.1 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.5             2000 Stock Option and Incentive Plan (filed as Exhibit 10.2 to
                the Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.6             2000 Non-Employee Director Plan (filed as Exhibit 10.4 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.7             2000 Employee Stock Purchase Plan (filed as Exhibit 10.3 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.8             Fifth Amended and Restated Investor Rights Agreement, by and
                among the registrant and the Investors and Founders listed
                therein, dated as of April 24, 2000, as amended by Amendment No.
                1 dated July 7, 2000 (filed as Exhibit 10.5 to the Registration
                Statement on Form S-1 (File No. 333-37316) and incorporated
                herein by reference)

5.1             Opinion of Testa, Hurwitz & Thibeault, LLP

23.1            Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit
                5.1)

23.2            Consent of Arthur Andersen LLP

24.1            Power of Attorney (included as part of the signature page of
                this Registration Statement)
[{"type": "S-8", "sequence": "1", "filename": "0001.txt", "description": "FORM S-8", "secsgml_size_bytes": 22807}]
{"company-data": {"conformed-name": "AVICI SYSTEMS INC", "cik": "0001094895", "assigned-sic": "3576", "irs-number": "020493372", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-48178", "film-number": "742196"}, "business-address": {"street1": "101 BILLERICA AVENUE", "city": "NORTH BILLERICA", "state": "MA", "zip": "01862", "phone": "9789642000"}, "mail-address": {"street1": "101 BILLERICA AVE", "city": "NORTH BILLERICA", "state": "MA", "zip": "01862"}}
As filed with the Securities and Exchange Commission on October 18, 2000
                                                      Registration No. 333-_____

================================================================================

                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549
                                 _____________

                                   FORM S-8

                         REGISTRATION STATEMENT UNDER
                          THE SECURITIES ACT OF 1933

                              AVICI SYSTEMS INC.
            (Exact Name of Registrant as specified in its charter)

               Delaware                                02-0493372
   (State or other jurisdiction of        (I.R.S. Employer Identification No.)
    incorporation or organization)

                             101 Billerica Avenue
                           North Billerica, MA 01862
                                (978) 964-2000
              (Address of Principal Executive Offices) (Zip Code)

                          __________________________

                              AVICI SYSTEMS INC.
                           1997 Stock Incentive Plan
                     2000 Stock Option and Incentive Plan
                 2000 Non-Employee Director Stock Option Plan
                       2000 Employee Stock Purchase Plan
                           (Full title of the plans)

                          __________________________

                            Peter C. Anastos, Esq.
                              Avici Systems Inc.
                             101 Billerica Avenue
                           North Billerica, MA 01862
              (Name and Address of Agent for Service of Process)

                                (978) 964-2000
         (Telephone Number, Including Area Code, of Agent For Service)

                          __________________________

                                   Copy to:

                             John A. Meltaus, Esq.
                        Testa, Hurwitz & Thibeault, LLP
                                125 High Street
                          Boston, Massachusetts 02110
                                (617) 248-7000

================================================================================

================================================================================

                        CALCULATION OF REGISTRATION FEE

================================================================================
Proposed
                                                                   Maximum        Proposed
                                                                   Offering       Maximum
Title of Securities                                 Amount to be  Price Per      Aggregate         Amount of
to be Registered                                     Registered     Share      Offering Price    Registration
                                                     ----------     -----      --------------    ------------
                                                                                                    Fee(1)
                                                                                                    ------
1997 Stock Incentive Plan
Common Stock (par value $.0001 per share)             150,000(2)   $  0.01    $      1,500.00     $      0.40
                                                       97,125(2)   $  0.10    $      9,712.50     $      2.56
                                                      198,999(2)   $  0.50    $     99,499.50     $     26.27
                                                      464,894(2)   $  1.00    $    464,894.00     $    122.73
                                                      542,179(2)   $  2.00    $  1,084,358.00     $    286.27
                                                      220,669(2)   $  3.00    $    662,007.00     $    174.77
                                                    3,455,604(2)   $  4.00    $ 13,822,416.00     $  3,649.12
                                                      887,500(2)   $ 12.00    $ 10,650,000.00     $  2,811.60
                                                      867,200(2)   $ 15.00    $ 13,008,000.00     $  3,434.11
                                                      440,000(2)   $ 25.00    $ 11,000,000.00     $  2,904.00
                                                      400,000(2)   $ 31.00    $ 12,400,000.00     $  3,273.60
                                                      200,000(2)   $ 96.75    $ 19,350,000.00     $  5,108.40

2000 Stock Option and Incentive Plan
Common Stock (par value $.0001 per share)             201,000(2)   $ 88.38    $ 17,764,380.00     $  4,689.80
                                                      130,000(2)   $102.00    $ 13,260,000.00     $  3,500.64
                                                       42,500(2)   $115.98    $  4,929,150.00     $  1,301.30
                                                       14,000(2)   $116.06    $  1,624,840.00     $    428.96
                                                          700(2)   $133.00    $     93,100.00     $     24.58
                                                    2,770,593(3)   $ 83.19    $230,485,631.67     $ 60,848.21

2000 Non-Employee Director Stock
Option Plan
Common Stock (par value $.0001 per share)             200,000(2)   $ 31.00    $  6,200,000.00     $  1,636.80
                                                      200,000(3)   $ 83.19    $ 16,638,000.00     $  4,392.43

2000 Employee Stock Purchase Plan                     750,000(3)   $ 83.19    $ 62,392,500.00     $ 16,471.62
Common Stock (par value $.0001 per share)

Total                                              12,232,963                 $435,939,988.67     $115,088.16
-------------------------------------------------------------------------------------------------------------
(1)  Calculated pursuant to Section 6(b) of the Securities Act of 1933, as
     amended.

(2)  Such shares are issuable upon the exercise of outstanding options with
     fixed exercise prices. Pursuant to Rule 457(h), the aggregate offering
     price and the fee have been computed upon the basis of the price at which
     the options may be exercised. The offering price per share set forth for
     such shares is the exercise price per share at which such options are
     exercisable.

(3)  The price of $83.19 per share, which is the average of the high and low
     price of the Common Stock of the Registrant as reported on the Nasdaq
     National Market on October 17, 2000, is set forth solely for purposes of
     calculating the filing fee pursuant to Rule 457(c) of the Securities Act
     and has been used only for those shares without a fixed exercise price.
PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

Item 1. Plan Information.
        ----------------

        The documents containing the information specified in this Item 1 will
be sent or given to employees, directors or others as specified by Rule
428(b)(1). In accordance with the rules and regulations of the Securities and
Exchange Commission (the "Commission") and the instructions to Form S-8, such
documents are not being filed with the Commission either as part of this
Registration Statement or as prospectuses or prospectus supplements pursuant to
Rule 424.

Item 2. Registrant Information and Employee Plan Annual Information.
        -----------------------------------------------------------

        The documents containing the information specified in this Item 2 will
be sent or given to employees, directors or others as specified by Rule 428(b).
In accordance with the rules and regulations of the Commission and the
instructions to Form S-8, such documents are not being filed with the Commission
either as part of this Registration Statement or as prospectuses or prospectus
supplements pursuant to Rule 424.

                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3. Incorporation of Documents by Reference.
        ---------------------------------------

        The following documents filed with the Commission are incorporated by
reference in this Registration Statement:

(a)     Registrant's Prospectus dated July 27, 2000, as filed with the
        Commission pursuant to Rule 424(b)(1) of the Securities Act of 1933, as
        amended (the "Securities Act of 1933"), on July 27, 2000;

(b)     The section entitled "Description of Registrant's Securities to be
        Registered" contained in the Registrant's Registration Statement on Form
        8-A (File No. 000-30865) filed on June 21, 2000 pursuant to Section
        12(g) of the Securities Exchange Act of 1934, as amended (the "Exchange
        Act").

(c)     The Registrant's Quarterly Report on Form 10-Q for the fiscal quarter
        ended June 30, 2000 filed on August 31, 2000 pursuant to the Exchange
        Act.

        All documents subsequently filed with the Commission by the Registrant
pursuant to Sections 13(a), 13(c), 14 and 15(d) of the Exchange Act, prior to
the filing of a post-effective amendment which indicates that all securities
offered herein have been sold or which deregisters all securities then remaining
unsold, shall be deemed to be incorporated by reference in this Registration
Statement and to be a part thereof from the date of filing of such documents.

Item 4. Description of Securities.
        -------------------------

        Not applicable.
Item 5. Interest of Named Experts and Counsel.
        -------------------------------------

        Not applicable.

Item 6. Indemnification of Directors and Officers.
        -----------------------------------------

        The Delaware General Corporation Law and the Registrant's charter
provide for indemnification of the Registrant's directors and officers for
liabilities and expenses that they may incur in such capacities. In general,
directors and officers are indemnified with respect to actions taken in good
faith in a manner reasonably believed to be in, or not opposed to, the best
interests of the Registrant, and with respect to any criminal action or
proceeding, actions that the indemnitee had no reasonable cause to believe were
unlawful.

        The underwriting agreement between the Registrant and the underwriters
listed therein provides that the underwriters are obligated, under certain
circumstances, to indemnify directors, officers and controlling persons of the
Registrant against certain liabilities, including liabilities under the
Securities Act of 1933. Reference is made to the form of underwriting agreement
filed as Exhibit 1.1 to the Registrant's Registration Statement on Form S-1
(File No. 333-37316).

        The Registrant has in effect a directors' and officers' insurance
policy.

Item 7. Exemption From Registration Claimed.
        -----------------------------------

        Not applicable.

Item 8. Exhibits.
        --------

   Exhibit No.  Description
   -----------  -----------

   4.1          Specimen Certificate for shares of the Registrant's Common Stock
                (filed as Exhibit 4.1 to the Registrant's Registration Statement
                on Form S-1 (File No. 333-37316) and incorporated herein by
                reference)

   4.2          Fourth Restated Certificate of Incorporation of the Registrant
                (filed as Exhibit 3.2 to the Registration Statement on Form S-1
                (File No. 333-37316) and incorporated herein by reference)

   4.3          Amended and Restated By-Laws of the Registrant (filed as Exhibit
                3.4 to the Registration Statement on Form S-1 (File No. 333-
                37316) and incorporated herein by reference)

   4.4          1997 Stock Incentive Plan (filed as Exhibit 10.1 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.5          2000 Stock Option and Incentive Plan (filed as Exhibit 10.2 to
                the Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.6          2000 Non-Employee Director Plan (filed as Exhibit 10.4 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by
reference)

   4.7          2000 Employee Stock Purchase Plan (filed as Exhibit 10.3 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

   4.8          Fifth Amended and Restated Investor Rights Agreement, by and
                among the registrant and the Investors and Founders listed
                therein, dated as of April 24, 2000, as amended by Amendment No.
                1 dated July 7, 2000 (filed as Exhibit 10.5 to the Registration
                Statement on Form S-1 (File No. 333-37316) and incorporated
                herein by reference)

   5.1          Opinion of Testa, Hurwitz & Thibeault, LLP

   23.1         Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit
                5.1)

   23.2         Consent of Arthur Andersen LLP

   24.1         Power of Attorney (included as part of the signature page of
                this Registration Statement)

Item 9. Undertakings.
        ------------

        The undersigned Registrant hereby undertakes to file, during any period
in which offers or sales are being made, a post-effective amendment to this
registration statement:

          (i)    to include any prospectus required by Section 10(a)(3) of the
        Securities Act of 1933;

          (ii)   to reflect in the prospectus any facts or events arising after
        the effective date of the registration statement (or the most recent
        post-effective amendment thereof) which, individually or in the
        aggregate, represent a fundamental change in the information set forth
        in the registration statement; and

          (iii)  to include any material information with respect to the plan of
        distribution not previously disclosed in the registration statement or
        any material change to such information in the registration statement;

        provided, however, that paragraphs (i) and (ii) above do not apply if
        the Registration Statement is on Form S-3, Form S-8 or Form F-3, and the
        information required to be included in a post-effective amendment by
        those paragraphs is contained in periodic reports filed with or
        furnished to the Commission by the Registrant pursuant to Section 13 or
        15(d) of the Exchange Act that are incorporated by reference in the
        registration statement.

        The undersigned Registrant hereby undertakes (i) that, for the purpose
of determining any liability under the Securities Act of 1933, each such post-
effective amendment shall be deemed to be a new registration statement relating
to the securities offered therein, and the offering of such securities at that
time shall be deemed to be the initial bona fide offering thereof; and (ii) to
remove from registration by means of a post-effective amendment any of the
securities being registered which remain unsold at the termination of the
offering.
The undersigned Registrant hereby undertakes that, for purposes of
determining any liability under the Securities Act of 1933, each filing of the
Registrant's annual report pursuant to Section 13(a) or 15(d) of the Exchange
Act that is incorporated by reference herein shall be deemed to be a new
registration statement relating to the securities offered therein, and the
offering of such securities at that time shall be deemed to be the initial bona
fide offering thereof.

     Insofar as indemnification for liabilities arising under the Securities Act
of 1933 may be permitted to directors, officers and controlling persons of the
Registrant pursuant to the foregoing provisions, or otherwise, the Registrant
has been advised that in the opinion of the Commission such indemnification is
against public policy as expressed in the Securities Act of 1933 and is,
therefore, unenforceable. In the event that a claim for indemnification against
such liabilities (other than the payment by the Registrant of expenses incurred
or paid by a director, officer or controlling person of the Registrant in the
successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the Registrant will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act of 1933 and will be governed by the
final adjudication of such issue.
SIGNATURES

     Pursuant to the requirements of the Securities Act of 1933, the Registrant
certifies that it has reasonable grounds to believe that it meets all of the
requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Billerica in the State of Massachusetts, on this 18th
day of October, 2000.

                                    AVICI SYSTEMS INC.

                                    By:  /s/ Paul F. Brauneis
                                         --------------------
                                         Paul F. Brauneis
                                         Chief Financial Officer and Treasurer

                       POWER OF ATTORNEY AND SIGNATURES

     We, the undersigned officers and directors of Avici Systems Inc., hereby
severally constitute and appoint Paul F. Brauneis, our true and lawful attorney,
with full power to him, to sign for us in our names in the capacities indicated
below, any amendments to this Registration Statement on Form S-8 (including
post-effective amendments), and to file the same, with all exhibits thereto and
other documents in connection therewith, with the Securities and Exchange
Commission, and generally to do all things in our names and on our behalf in our
capacities as officers and directors to enable Avici Systems Inc., to comply
with the provisions of the Securities Act of 1933, as amended, hereby ratifying
and confirming our signatures as they may be signed by our said attorneys, or
any of them, to said Registration Statement and all amendments thereto.

     Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the date indicated.

             SIGNATURE                                         TITLE

/s/ Surya R. Panditi                  President, Chief Executive Officer
---------------------------           and Director (principal executive officer)
Surya R. Panditi

/s/ Steven B. Kaufman                 Director and Chief Operating Officer
---------------------------
Steven B. Kaufman

/s/ Paul F. Brauneis                  Chief Financial Officer, Treasurer and
---------------------------           Secretary (principal financial officer and
Paul F. Brauneis                      principal accounting officer)

/s James R. Swartz                    Chairman and Director
---------------------------
James R. Swartz

/s/ Stephen M. Diamond                Director
---------------------------
Stephen M. Diamond

/s/ Catherine M. Hapka                Director
---------------------------
Catherine M. Hapka

/s/ Richard T. Liebhaber              Director
---------------------------
Richard T. Liebhaber

/s/ James Mongiello                   Director
---------------------------
James Mongiello

/s/ Henry Zannini                     Director
---------------------------
Henry Zannini
INDEX TO EXHIBITS

Exhibit No.     Description
-----------     -----------

4.1             Specimen Certificate for shares of the Registrant's Common Stock
                (filed as Exhibit 4.1 to the Registrant's Registration Statement
                on Form S-1 (File No. 333-37316) and incorporated herein by
                reference)

4.2             Fourth Restated Certificate of Incorporation of the Registrant
                (filed as Exhibit 3.2 to the Registration Statement on Form S-1
                (File No. 333-37316) and incorporated herein by reference)

4.3             Amended and Restated By-Laws of the Registrant (filed as Exhibit
                3.4 to the Registration Statement on Form S-1 (File No. 333-
                37316) and incorporated herein by reference)

4.4             1997 Stock Incentive Plan (filed as Exhibit 10.1 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.5             2000 Stock Option and Incentive Plan (filed as Exhibit 10.2 to
                the Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.6             2000 Non-Employee Director Plan (filed as Exhibit 10.4 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.7             2000 Employee Stock Purchase Plan (filed as Exhibit 10.3 to the
                Registration Statement on Form S-1 (File No. 333-37316) and
                incorporated herein by reference)

4.8             Fifth Amended and Restated Investor Rights Agreement, by and
                among the registrant and the Investors and Founders listed
                therein, dated as of April 24, 2000, as amended by Amendment No.
                1 dated July 7, 2000 (filed as Exhibit 10.5 to the Registration
                Statement on Form S-1 (File No. 333-37316) and incorporated
                herein by reference)

5.1             Opinion of Testa, Hurwitz & Thibeault, LLP

23.1            Consent of Testa, Hurwitz & Thibeault, LLP (included in Exhibit
                5.1)

23.2            Consent of Arthur Andersen LLP

24.1            Power of Attorney (included as part of the signature page of
                this Registration Statement)