<PAGE>

   As filed with the Securities and Exchange Commission on November 22, 1999
                                                     Registration No. 333-____
--------------------------------------------------------------------------------

                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                   FORM S-8
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933

                              CAIS INTERNET, INC.
            (Exact Name of Registrant as Specified in Its Charter)

                   Delaware                               52-2066769
        (State or Other Jurisdiction of                (I.R.S. Employer
         Incorporation or Organization)              Identification Number)

                            1255 22nd Street, N.W.
                            Washington, D.C. 20037
          (Address of Principal Executive Office, Including Zip Code)

      CAIS INTERNET, INC. 1998 AMENDED AND RESTATED EQUITY INCENTIVE PLAN
                                      AND
               ATCOM, INC. 1996 STOCK OPTION/STOCK ISSUANCE PLAN
                           (Full Title of the Plans)

                             Ulysses G. Auger, II
                     Chief Executive Officer and Chairman
                              CAIS Internet, Inc.
                            1255 22nd Street, N.W.
                            Washington, D.C. 20037
                    (Name and Address of Agent for Service)

                                (202) 715-1300
         (Telephone Number, Including Area Code, of Agent for Service)

                                   Copy to:
                              Morris DeFeo, Esq.
                            Morrison & Foerster LLP
                   2000 Pennsylvania Avenue, NW, Suite 5500
                          Washington, D.C. 20006-1888
<TABLE>
<CAPTION>
====================================================================================================================================
                                                  CALCULATION OF REGISTRATION FEE
====================================================================================================================================
                                                         Proposed
       Title of                                           maximum                 Proposed maximum
   securities to be             Amount to be         offering price per      aggregate offering price               Amount of
      registered               registered (1)           share (2)                      (2)                       registration fee
------------------------------------------------------------------------------------------------------------------------------------
<S>                            <C>                   <C>                     <C>                                 <C>
Common Stock, $0.01              3,500,000             $ 14.31                        $ 50,085,000                  $ 13,923.63
par value per share
====================================================================================================================================
</TABLE>

(1) The number of shares of CAIS Internet, Inc. Common Stock to be issued gives
    effect, in part, to the exchange ratio of 0.3553 shares of CAIS Internet,
    Inc. Common Stock for each share of Atcom, Inc. Common Stock originally
    issuable under the Atcom, Inc. 1996 Stock Option/Stock Issuance Plan, as
    provided in the Amended and Restated Agreement and Plan of Merger dated
    August 4, 1999 by and among CAIS Internet, Inc., CIAM Corp. and Atcom, Inc.
(2) Estimated solely for purposes of calculating the registration fee, and based
    upon the fixed exercise prices of outstanding options in accordance with
    Rule 457(h) under the Securities Act of 1933, as amended (the "Securities
    Act").
(3) Estimated solely for the purpose of calculating the registration fee in
    accordance with Rule 457(h) and Rule 457(c) under the Securities Act of
    1933, based upon an average of the high and low prices of CAIS Internet,
    Inc. common stock reported on the Nasdaq National Market on November 18,
    1999.
<PAGE>

                               EXPLANATORY NOTE

     This registration statement has been prepared in accordance with the
requirements of Form S-8 under the Securities Act to register 3,500,000 shares
of the Common Stock of CAIS Internet, Inc., a Delaware corporation (the
"Registrant"), issuable to eligible employees, directors, consultants and
advisors under the Registrant's 1998 Amended and Restated Equity Incentive Plan
and the Atcom, Inc. 1996 Stock Option/Stock Issuance Plan assumed by Registrant
upon the merger of Registrant's wholly owned subsidiary, CIAM Corp., a
California corporation, with and into Atcom, Inc., a California corporation,
which plan was previously an obligation of Atcom, Inc.

                                    PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

     The documents containing the information specified in Part 1 of Form S-8
(plan information and registrant information and employee plan annual
information) will be sent or given to employees as specified by Securities and
Exchange Commission Rule 428(b)(1). Such documents need not be filed with the
Securities and Exchange Commission either as part of this Registration Statement
or as prospectuses or prospectus supplements pursuant to Rule 424. These
documents and the documents incorporated by reference in this Registration
Statement pursuant to Item 3 of Form S-8 (Part II hereof), taken together,
constitute a prospectus that meets the requirements of Section 10(a) of the
Securities Act of 1933.

                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

   The following documents filed by CAIS Internet, Inc. (the "Registrant")
with the Securities and Exchange Commission (the "Commission") are incorporated
by reference herein:

   (a)  The Registrant's prospectus dated May 20, 1999 filed pursuant to Rule
   424(b) under the Securities Act, which contains audited financial statements
   for the Registrant's latest fiscal year for which such statements have been
   filed.

   (b)  All reports filed pursuant to Sections 13(a) or 15(d) of the Exchange
   Act since the end of the fiscal year covered by the prospectus referred to in
   (1) above.

   (c)  The description of the common stock of the Registrant, $.01 par value
   per share (the "Common Stock"), contained in the registration statement filed
   as of May 17, 1999 under Section 12 of the Exchange Act, including any
   amendment or report filed for purposes of updating such description.

   All documents filed by the Registrant with the Commission pursuant to Section
13(a), 13(c), 14 or 15(d) of the Exchange Act, prior to the filing of a post-
effective amendment which indicates that all securities offered have been sold
or which deregisters all securities then remaining unsold, shall be deemed to be
incorporated by reference into this Registration Statement and to be a part
hereof from the date of filing of such documents. Any statement contained in a
document incorporated by reference herein shall be deemed to be modified or
superseded for purposes of this Registration Statement to the extent that

                                       2
<PAGE>

a statement contained herein or in any other subsequently filed document which
also is or is deemed to be incorporated by reference herein modifies or
supersedes such statement. Any statement so modified or superseded shall not be
deemed, except as so modified or superseded, to constitute a part of this
Registration Statement.

Item 4.  Description of Securities.

     Not applicable.

Item 5.  Interests of Named Experts and Counsel.

     Not applicable.

Item 6.  Indemnification of Directors and Officers.

     Reference is made to Section 102(b)(7) of the Delaware General Corporation
Law (the "DGCL"), which permits a corporation in its certificate of
incorporation or an amendment thereto to eliminate or limit the personal
liability of a director for violations of the director's fiduciary duty, except
(1) for any breach of the director's fiduciary duty of loyalty to the
corporation or its stockholders, (2) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (3) pursuant
to Section 174 of the DGCL (providing for liability of directors for unlawful
payment of dividends or unlawful stock purchases or redemptions), or (4) for any
transaction from which the director derived an improper personal benefit. The
Registrant's Certificate of Incorporation contains provisions permitted by
Section 102(b)(7) of the DGCL.

     Reference is made to Section 145 of the DGCL which provides that a
corporation may indemnify any persons, including directors and officers, who
are, or are threatened to be made, parties to any threatened, pending or
completed legal action, suit or proceeding, whether civil, criminal,
administrative or investigative (other than an action by or in the right of such
corporation), by reason of the fact that such person is or was a director,
officer, employee or agent of such corporation, or is or was serving at the
request of such corporation as a director, officer, employee or agent of another
corporation or enterprise. The indemnity may include expenses (including
attorney's fees), judgments, fines and amounts paid in settlement actually and
reasonably incurred by such person in connection with such action, suit or
proceeding, provided such director, officer, employee or agent acted in good
faith and in a manner he reasonably believed to be in or not opposed to the
corporation's best interests and, with respect to any criminal actions or
proceedings, had no reasonable cause to believe that his conduct was unlawful. A
Delaware corporation may indemnify directors and/or officers in an action or
suit by or in the right of the corporation under the same conditions, except
that no indemnification is permitted without judicial approval if the director
or officer is adjudged to be liable to the corporation. Where a director or
officer is successful on the merits or otherwise in the defense of any action
referred to above, the corporation must indemnify him or her against the
expenses which such director or officer actually and reasonably incurred.

     The Registrant's Certificate of Incorporation provides indemnification of
directors and officers of the Registrant to the fullest extent permitted by the
DGCL.

Item 7.  Exemption From Registration Claimed.

     Not applicable.

                                       3
<PAGE>

Item 8.  Exhibits.

     The Exhibit Index immediately preceding the exhibits is incorporated herein
by reference.

Item 9.  Undertakings.

    a.    The Registrant hereby undertakes:


    (1)   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;

    (ii)  To reflect in the prospectus any facts or events arising after the
    effective date of this 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. Notwithstanding the foregoing, any increase or
    decrease in volume of securities offered (if the total dollar value of
    securities offered would not exceed that which was registered) and any
    deviation from the low or high end of the estimated maximum offering range
    may be reflected in the form of prospectus filed with the Commission
    pursuant to Rule 424(b) if, in the aggregate, the changes in volume and
    price represent no more than 20 percent change in the maximum aggregate
    offering price set forth in the "Calculation of Registration Fee" table
    herein; and

    (iii) To include any material information with respect to the plan of
    distribution not previously disclosed in this Registration Statement or any
    material change to such information in this Registration Statement;

    provided, however, that paragraphs (i) and (ii) do not apply if 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 Section 15(d) of the
    Exchange Act that are incorporated by reference in this Registration
    Statement.

    (2)   That, for the purpose of determining any liability under the
    Securities Act, 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.

    (3)   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.

    b.    The Registrant hereby undertakes that, for purposes of determining any
    liability under the Securities Act, each filing of the Registrant's annual
    report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and,
    where applicable, each filing of an employee benefit plan's annual report
    pursuant to Section 15(d) of the Exchange Act) that is incorporated by
    reference in this Registration Statement 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 in the
    initial bona fide offering thereof.

    c.    Insofar as indemnification for liabilities arising under the
    Securities Act 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
    Securities and Exchange Commission such indemnification is against public
    policy as expressed in the Act 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 and will be governed by the
    final adjudication of such issue.

                                       4
<PAGE>

                                  SIGNATURES
                                  ----------

     Pursuant to the requirements of the Securities Act of 1933, the Registrant,
CAIS Internet, Inc., 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 Washington, D.C., on November 22, 1999.

                                    CAIS INTERNET, INC.


                                    By:  /s/ Ulysses G. Auger, II
                                         ------------------------
                                         Ulysses G. Auger, II
                                         President and Chief Executive Officer

                               POWER OF ATTORNEY
                               -----------------

     Each person whose signature appears below constitutes and appoints Ulysses
G. Auger, II, and Michael Plantamura, and each of them, as attorneys-in-fact,
each with the power of substitution, for him in any and all capacities, to sign
any amendment to this Registration Statement and to file the same, with exhibits
thereto and other documents in connection therewith, with the Securities and
Exchange Commission, granting to said attorneys-in-fact, full power and
authority to do and perform each and every act and thing requisite and necessary
to be done in connection therewith, as fully to all intents and purposes as he
might or could do in person, hereby ratifying and confirming the said attorney-
in-fact or his substitute or substitutes, may lawfully do or cause to be done by
virtue hereof.

     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.

<TABLE>
<CAPTION>
   Signature                            Capacity                      Date
   ---------                            --------                      ----
   <S>                                  <C>                           <C>
    /s/ Ulysses G. Auger, II            President, Chief              November 22, 1999
   ------------------------------
    Ulysses G. Auger, II                Executive Officer and
                                        Director (Principal
                                        Executive Officer)


    /s/ Barton R. Groh                  Chief Financial Officer       November 22, 1999
   ------------------------------
    Barton R. Groh                      (Principal Financial
                                        and Accounting Officer)



    /s/ Ulysses G. Auger, Sr.           Director                      November 22, 1999
   ------------------------------
    Ulysses G. Auger, Sr.


    /s/ William M. Caldwell, IV         Director                      November 22, 1999
   ------------------------------
    William M. Caldwell, IV


    /s/ Richard F. Levin
   ------------------------------       Director                      November 22, 1999
    Richard F. Levin


    /s/ Vernon Fotheringham             Director                      November 22, 1999
   ------------------------------
    Vernon Fotheringham


    /s/ S. Theodore Ammon               Director                      November 22, 1999
   ------------------------------
    S. Theodore Ammon
</TABLE>

                                       5
<PAGE>

                               INDEX TO EXHIBITS
                               -----------------


Exhibit
Number                   Description

5.1    Opinion of Morrison & Foerster LLP

23.1   Consent of Morrison & Foerster LLP (included in Exhibit 5.1)

23.2   Consent of Deloitte & Touche LLP

23.3   Consent of Arthur Andersen LLP

24.1   Power of Attorney (included on the signature page of this Registration
       Statement)
[{"type": "S-8", "sequence": "1", "description": "FORM S-8", "secsgml_size_bytes": 19272}]
{"company-data": {"conformed-name": "CAIS INTERNET INC", "cik": "0001078404", "assigned-sic": "7374", "irs-number": "522066769", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-91403", "film-number": "99761888"}, "business-address": {"street1": "1255 22ND STREET N W", "street2": "FOURTH FLOOR", "city": "WASHINGTON", "state": "DC", "zip": "20037", "phone": "2027151300"}}
As filed with the Securities and Exchange Commission on November 22, 1999
                                                     Registration No. 333-____
--------------------------------------------------------------------------------

                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                   FORM S-8
                            REGISTRATION STATEMENT
                                     UNDER
                          THE SECURITIES ACT OF 1933

                              CAIS INTERNET, INC.
            (Exact Name of Registrant as Specified in Its Charter)

                   Delaware                               52-2066769
        (State or Other Jurisdiction of                (I.R.S. Employer
         Incorporation or Organization)              Identification Number)

                            1255 22nd Street, N.W.
                            Washington, D.C. 20037
          (Address of Principal Executive Office, Including Zip Code)

      CAIS INTERNET, INC. 1998 AMENDED AND RESTATED EQUITY INCENTIVE PLAN
                                      AND
               ATCOM, INC. 1996 STOCK OPTION/STOCK ISSUANCE PLAN
                           (Full Title of the Plans)

                             Ulysses G. Auger, II
                     Chief Executive Officer and Chairman
                              CAIS Internet, Inc.
                            1255 22nd Street, N.W.
                            Washington, D.C. 20037
                    (Name and Address of Agent for Service)

                                (202) 715-1300
         (Telephone Number, Including Area Code, of Agent for Service)

                                   Copy to:
                              Morris DeFeo, Esq.
                            Morrison & Foerster LLP
                   2000 Pennsylvania Avenue, NW, Suite 5500
                          Washington, D.C. 20006-1888
====================================================================================================================================
                                                  CALCULATION OF REGISTRATION FEE
====================================================================================================================================
                                                         Proposed
       Title of                                           maximum                 Proposed maximum
   securities to be             Amount to be         offering price per      aggregate offering price               Amount of
      registered               registered (1)           share (2)                      (2)                       registration fee
------------------------------------------------------------------------------------------------------------------------------------
Common Stock, $0.01              3,500,000             $ 14.31                        $ 50,085,000                  $ 13,923.63
par value per share
====================================================================================================================================
(1) The number of shares of CAIS Internet, Inc. Common Stock to be issued gives
    effect, in part, to the exchange ratio of 0.3553 shares of CAIS Internet,
    Inc. Common Stock for each share of Atcom, Inc. Common Stock originally
    issuable under the Atcom, Inc. 1996 Stock Option/Stock Issuance Plan, as
    provided in the Amended and Restated Agreement and Plan of Merger dated
    August 4, 1999 by and among CAIS Internet, Inc., CIAM Corp. and Atcom, Inc.
(2) Estimated solely for purposes of calculating the registration fee, and based
    upon the fixed exercise prices of outstanding options in accordance with
    Rule 457(h) under the Securities Act of 1933, as amended (the "Securities
    Act").
(3) Estimated solely for the purpose of calculating the registration fee in
    accordance with Rule 457(h) and Rule 457(c) under the Securities Act of
    1933, based upon an average of the high and low prices of CAIS Internet,
    Inc. common stock reported on the Nasdaq National Market on November 18,
    1999.
EXPLANATORY NOTE

     This registration statement has been prepared in accordance with the
requirements of Form S-8 under the Securities Act to register 3,500,000 shares
of the Common Stock of CAIS Internet, Inc., a Delaware corporation (the
"Registrant"), issuable to eligible employees, directors, consultants and
advisors under the Registrant's 1998 Amended and Restated Equity Incentive Plan
and the Atcom, Inc. 1996 Stock Option/Stock Issuance Plan assumed by Registrant
upon the merger of Registrant's wholly owned subsidiary, CIAM Corp., a
California corporation, with and into Atcom, Inc., a California corporation,
which plan was previously an obligation of Atcom, Inc.

                                    PART I

             INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

     The documents containing the information specified in Part 1 of Form S-8
(plan information and registrant information and employee plan annual
information) will be sent or given to employees as specified by Securities and
Exchange Commission Rule 428(b)(1). Such documents need not be filed with the
Securities and Exchange Commission either as part of this Registration Statement
or as prospectuses or prospectus supplements pursuant to Rule 424. These
documents and the documents incorporated by reference in this Registration
Statement pursuant to Item 3 of Form S-8 (Part II hereof), taken together,
constitute a prospectus that meets the requirements of Section 10(a) of the
Securities Act of 1933.

                                    PART II

              INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

Item 3.  Incorporation of Documents by Reference.

   The following documents filed by CAIS Internet, Inc. (the "Registrant")
with the Securities and Exchange Commission (the "Commission") are incorporated
by reference herein:

   (a)  The Registrant's prospectus dated May 20, 1999 filed pursuant to Rule
   424(b) under the Securities Act, which contains audited financial statements
   for the Registrant's latest fiscal year for which such statements have been
   filed.

   (b)  All reports filed pursuant to Sections 13(a) or 15(d) of the Exchange
   Act since the end of the fiscal year covered by the prospectus referred to in
   (1) above.

   (c)  The description of the common stock of the Registrant, $.01 par value
   per share (the "Common Stock"), contained in the registration statement filed
   as of May 17, 1999 under Section 12 of the Exchange Act, including any
   amendment or report filed for purposes of updating such description.

   All documents filed by the Registrant with the Commission pursuant to Section
13(a), 13(c), 14 or 15(d) of the Exchange Act, prior to the filing of a post-
effective amendment which indicates that all securities offered have been sold
or which deregisters all securities then remaining unsold, shall be deemed to be
incorporated by reference into this Registration Statement and to be a part
hereof from the date of filing of such documents. Any statement contained in a
document incorporated by reference herein shall be deemed to be modified or
superseded for purposes of this Registration Statement to the extent that

                                       2
a statement contained herein or in any other subsequently filed document which
also is or is deemed to be incorporated by reference herein modifies or
supersedes such statement. Any statement so modified or superseded shall not be
deemed, except as so modified or superseded, to constitute a part of this
Registration Statement.

Item 4.  Description of Securities.

     Not applicable.

Item 5.  Interests of Named Experts and Counsel.

     Not applicable.

Item 6.  Indemnification of Directors and Officers.

     Reference is made to Section 102(b)(7) of the Delaware General Corporation
Law (the "DGCL"), which permits a corporation in its certificate of
incorporation or an amendment thereto to eliminate or limit the personal
liability of a director for violations of the director's fiduciary duty, except
(1) for any breach of the director's fiduciary duty of loyalty to the
corporation or its stockholders, (2) for acts or omissions not in good faith or
which involve intentional misconduct or a knowing violation of law, (3) pursuant
to Section 174 of the DGCL (providing for liability of directors for unlawful
payment of dividends or unlawful stock purchases or redemptions), or (4) for any
transaction from which the director derived an improper personal benefit. The
Registrant's Certificate of Incorporation contains provisions permitted by
Section 102(b)(7) of the DGCL.

     Reference is made to Section 145 of the DGCL which provides that a
corporation may indemnify any persons, including directors and officers, who
are, or are threatened to be made, parties to any threatened, pending or
completed legal action, suit or proceeding, whether civil, criminal,
administrative or investigative (other than an action by or in the right of such
corporation), by reason of the fact that such person is or was a director,
officer, employee or agent of such corporation, or is or was serving at the
request of such corporation as a director, officer, employee or agent of another
corporation or enterprise. The indemnity may include expenses (including
attorney's fees), judgments, fines and amounts paid in settlement actually and
reasonably incurred by such person in connection with such action, suit or
proceeding, provided such director, officer, employee or agent acted in good
faith and in a manner he reasonably believed to be in or not opposed to the
corporation's best interests and, with respect to any criminal actions or
proceedings, had no reasonable cause to believe that his conduct was unlawful. A
Delaware corporation may indemnify directors and/or officers in an action or
suit by or in the right of the corporation under the same conditions, except
that no indemnification is permitted without judicial approval if the director
or officer is adjudged to be liable to the corporation. Where a director or
officer is successful on the merits or otherwise in the defense of any action
referred to above, the corporation must indemnify him or her against the
expenses which such director or officer actually and reasonably incurred.

     The Registrant's Certificate of Incorporation provides indemnification of
directors and officers of the Registrant to the fullest extent permitted by the
DGCL.

Item 7.  Exemption From Registration Claimed.

     Not applicable.

                                       3
Item 8.  Exhibits.

     The Exhibit Index immediately preceding the exhibits is incorporated herein
by reference.

Item 9.  Undertakings.

    a.    The Registrant hereby undertakes:

    (1)   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;

    (ii)  To reflect in the prospectus any facts or events arising after the
    effective date of this 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. Notwithstanding the foregoing, any increase or
    decrease in volume of securities offered (if the total dollar value of
    securities offered would not exceed that which was registered) and any
    deviation from the low or high end of the estimated maximum offering range
    may be reflected in the form of prospectus filed with the Commission
    pursuant to Rule 424(b) if, in the aggregate, the changes in volume and
    price represent no more than 20 percent change in the maximum aggregate
    offering price set forth in the "Calculation of Registration Fee" table
    herein; and

    (iii) To include any material information with respect to the plan of
    distribution not previously disclosed in this Registration Statement or any
    material change to such information in this Registration Statement;

    provided, however, that paragraphs (i) and (ii) do not apply if 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 Section 15(d) of the
    Exchange Act that are incorporated by reference in this Registration
    Statement.

    (2)   That, for the purpose of determining any liability under the
    Securities Act, 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.

    (3)   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.

    b.    The Registrant hereby undertakes that, for purposes of determining any
    liability under the Securities Act, each filing of the Registrant's annual
    report pursuant to Section 13(a) or Section 15(d) of the Exchange Act (and,
    where applicable, each filing of an employee benefit plan's annual report
    pursuant to Section 15(d) of the Exchange Act) that is incorporated by
    reference in this Registration Statement 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 in the
    initial bona fide offering thereof.

    c.    Insofar as indemnification for liabilities arising under the
    Securities Act 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
    Securities and Exchange Commission such indemnification is against public
    policy as expressed in the Act 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 and will be governed by the
    final adjudication of such issue.

                                       4
SIGNATURES
                                  ----------

     Pursuant to the requirements of the Securities Act of 1933, the Registrant,
CAIS Internet, Inc., 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 Washington, D.C., on November 22, 1999.

                                    CAIS INTERNET, INC.

                                    By:  /s/ Ulysses G. Auger, II
                                         ------------------------
                                         Ulysses G. Auger, II
                                         President and Chief Executive Officer

                               POWER OF ATTORNEY
                               -----------------

     Each person whose signature appears below constitutes and appoints Ulysses
G. Auger, II, and Michael Plantamura, and each of them, as attorneys-in-fact,
each with the power of substitution, for him in any and all capacities, to sign
any amendment to this Registration Statement and to file the same, with exhibits
thereto and other documents in connection therewith, with the Securities and
Exchange Commission, granting to said attorneys-in-fact, full power and
authority to do and perform each and every act and thing requisite and necessary
to be done in connection therewith, as fully to all intents and purposes as he
might or could do in person, hereby ratifying and confirming the said attorney-
in-fact or his substitute or substitutes, may lawfully do or cause to be done by
virtue hereof.

     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                            Capacity                      Date
   ---------                            --------                      ----
/s/ Ulysses G. Auger, II            President, Chief              November 22, 1999
   ------------------------------
    Ulysses G. Auger, II                Executive Officer and
                                        Director (Principal
                                        Executive Officer)

    /s/ Barton R. Groh                  Chief Financial Officer       November 22, 1999
   ------------------------------
    Barton R. Groh                      (Principal Financial
                                        and Accounting Officer)

    /s/ Ulysses G. Auger, Sr.           Director                      November 22, 1999
   ------------------------------
    Ulysses G. Auger, Sr.

    /s/ William M. Caldwell, IV         Director                      November 22, 1999
   ------------------------------
    William M. Caldwell, IV

    /s/ Richard F. Levin
   ------------------------------       Director                      November 22, 1999
    Richard F. Levin

    /s/ Vernon Fotheringham             Director                      November 22, 1999
   ------------------------------
    Vernon Fotheringham

    /s/ S. Theodore Ammon               Director                      November 22, 1999
   ------------------------------
    S. Theodore Ammon
5
INDEX TO EXHIBITS
                               -----------------

Exhibit
Number                   Description

5.1    Opinion of Morrison & Foerster LLP

23.1   Consent of Morrison & Foerster LLP (included in Exhibit 5.1)

23.2   Consent of Deloitte & Touche LLP

23.3   Consent of Arthur Andersen LLP

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