<PAGE>   1
     As filed with the Securities and Exchange Commission on August 5, 1999

                                                   Registration No. 333-________

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

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D. C. 20549

                                    FORM S-8

                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933


                      SECURITY DYNAMICS TECHNOLOGIES, INC.
             (Exact Name of Registrant as Specified in Its Charter)

            DELAWARE                                         04-2916506
(State or Other Jurisdiction of                           (I.R.S. Employer
 Incorporation or Organization)                        Identification Number)

36 CROSBY DRIVE, BEDFORD, MASSACHUSETTS                        01730
(Address of Principal Executive Offices)                     (Zip Code)


                  1994 EMPLOYEE STOCK PURCHASE PLAN, AS AMENDED
                            (Full Title of the Plan)


                             ARTHUR W. COVIELLO, JR.
                      SECURITY DYNAMICS TECHNOLOGIES, INC.
                                 36 CROSBY DRIVE
                          BEDFORD, MASSACHUSETTS 01730
                     (Name and Address of Agent for Service)

                                 (781) 301-5000
          (Telephone Number, Including Area Code, of Agent for Service)




<PAGE>   2




                         CALCULATION OF REGISTRATION FEE

==============    ==========      ===========      ==============   ============
                                   Proposed          Proposed
   Title of                       Maximum            Maximum
  Securities        Amount        Offering          Aggregate        Amount of
    to be           to be           Price            Offering       Registration
  Registered      Registered      Per Share           Price             Fee
 ------------      --------       ---------         ------------     ----------

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

Common Stock,      600,000        $17.5625(1)      $10,537,500(1)      $2,930
$.01 par value     shares
==============    ==========      ===========      ==============   ============

(1)      Estimated solely for the purpose of calculating the registration fee,
         and based on the average of the high and low prices of the Common Stock
         on the Nasdaq National Market on July 30, 1999 in accordance with Rules
         457(c) and 457(h) under the Securities Act of 1933.

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



<PAGE>   3



PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

                  The information required by Part I is included in documents
sent or given to participants in the Registrant's 1994 Employee Stock Purchase
Plan, as amended, pursuant to Rule 428(b)(1) of the Securities Act of 1933, as
amended (the "Securities Act").


PART II.  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         Item 3.  INCORPORATION OF DOCUMENTS BY REFERENCE

                  The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 14 and 15(d) of the Securities Exchange Act of
1934, as amended (the "Exchange Act"), and in accordance therewith files
reports, proxy statements and other information with the Securities and Exchange
Commission (the "Commission"). The following documents, which are on file with
the Commission, are incorporated in this Registration Statement by reference:

                  (1) The Registrant's latest annual report filed pursuant to
         Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
         filed pursuant to Rule 424(b) under the Securities Act that contains
         audited financial statements for the Registrant's latest fiscal year
         for which such statements have been filed.

                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
         of the Exchange Act since the end of the fiscal year covered by the
         document referred to in (1) above.

                  (3) The description of the common stock of the Registrant,
         $.01 par value per share (the "Common Stock"), contained in a
         registration statement filed under the Exchange Act, including any
         amendment or report filed for the purpose of updating such description.

                  All documents subsequently filed 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 shares of Common Stock
offered hereby have been sold or which deregisters all shares of Common Stock
then remaining unsold, shall be deemed to be incorporated by reference herein
and to be part hereof from the date of the filing of such documents. Any
statement contained in a document incorporated or deemed to be incorporated by
reference herein shall be deemed to be modified or superseded for purposes of
this Registration Statement to the extent that 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.



                                      II-1

<PAGE>   4



         Item 4.  DESCRIPTION OF SECURITIES

                  Not applicable.


         Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL

                  The legality of the Common Stock being offered hereby will be
passed upon for the Company by Hale and Dorr LLP, Boston, Massachusetts. A
senior partner of Hale and Dorr LLP beneficially owns approximately 3,300 shares
of the Registrant's Common Stock.


         Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

                  Article EIGHTH of the Registrant's Third Restated Certificate
of Incorporation, as amended (the "Restated Certificate of Incorporation"),
provides that no director of the Registrant shall be personally liable for any
monetary damages for any breach of fiduciary duty as a director, except to the
extent that the Delaware General Corporation Law prohibits the elimination or
limitation of liability of directors for breach of fiduciary duty.

                  Article NINTH of the Registrant's Restated Certificate of
Incorporation provides that a director or officer of the Registrant (a) shall be
indemnified by the Registrant against all expenses (including attorneys' fees),
judgments, fines and amounts paid in settlement incurred in connection with any
litigation or other legal proceeding (other than an action by or in the right of
the Registrant) brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal action or proceeding, had no
reasonable cause to believe his conduct was unlawful and (b) shall be
indemnified by the Registrant against all expenses (including attorneys' fees)
and amounts paid in settlement incurred in connection with any action by or in
the right of the Registrant brought against him by virtue of his position as a
director or officer of the Registrant if he acted in good faith and in a manner
he reasonably believed to be in, or not opposed to, the best interests of the
Registrant, except that no indemnification shall be made with respect to any
matter as to which such person shall have been adjudged to be liable to the
Registrant, unless a court determines that, despite such adjudication but in
view of all of the circumstances, he is entitled to indemnification of such
expenses. Notwithstanding the foregoing, to the extent that a director or
officer has been successful, on the merits or otherwise, including, without
limitation, the dismissal of an action without prejudice, he is required to be
indemnified by the Registrant against all expenses (including attorneys' fees)
incurred in connection therewith. Expenses shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately determined that he is not entitled to indemnification for
such expenses.

                  Indemnification is required to be made unless the Registrant
determines that the applicable standard of conduct required for indemnification
has not been met. In the event of a determination by the Registrant that the
director or officer did not meet

                                      II-2

<PAGE>   5



the applicable standard of conduct required for indemnification, or if the
Registrant fails to make an indemnification payment within 60 days after such
payment is claimed by such person, such person is permitted to petition the
court to make an independent determination as to whether such person is entitled
to indemnification. As a condition precedent to the right of indemnification,
the director or officer must give the Registrant notice of the action for which
indemnity is sought and the Registrant has the right to participate in such
action or assume the defense thereof.

                  Article NINTH of the Registrant's Restated Certificate of
Incorporation further provides that the indemnification provided therein is not
exclusive, and provides that in the event that the Delaware General Corporation
Law is amended to expand the indemnification permitted to directors or officers
the Registrant must indemnify those persons to the fullest extent permitted by
such law as so amended.

                  Section 145 of the Delaware General Corporation Law, as
amended, provides that a corporation may indemnify any person who was or is a
party or is threatened to be made a party to any threatened, pending or
completed action, suit or proceeding, whether civil, criminal, administrative or
investigative (other than an action by or in the right of the corporation) by
reason of the fact that he is or was a director, officer, employee or agent of
the corporation, or is or was serving at the request of the corporation as a
director, officer, employee or agent of another corporation, partnership, joint
venture, trust or other enterprise, against expenses (including attorneys'
fees), judgments, fines and amounts paid in settlement actually and reasonably
incurred by him in connection with such action, suit or proceeding if he acted
in good faith and in a manner he reasonably believed to be in or not opposed to
the best interests of the corporation, and, with respect to any criminal action
or proceeding, had no reasonable cause to believe his conduct was unlawful.
Section 145 further provides that a corporation similarly may indemnify any
person who was or is a party or is threatened to be made a party to any
threatened, pending or completed action or suit by or in the right of the
corporation to procure a judgment in its favor by reason of the fact that he is
or was a director, officer, employee or agent of the corporation, or is or was
serving at the request of the corporation as a director, officer, employee or
agent of another corporation, partnership, joint venture, trust or other
enterprise, against expenses (including attorneys' fees) actually and reasonably
incurred by him in connection with the defense or settlement of such action or
suit if he acted in good faith and in a manner he reasonably believed to be in
or not opposed to the best interests of the corporation, except that no
indemnification shall be made in respect of any claim, issue or matter as to
which such person shall have been adjudged to be liable to the corporation
unless and only to the extent that the Delaware Court of Chancery or the court
in which such action or suit was brought shall determine upon application that,
despite an adjudication of liability but in view of all the circumstances of the
case, such person is fairly and reasonably entitled to indemnity for such
expenses which the Court of Chancery or such other court shall deem proper.

                  The Registrant has directors and officers liability insurance
for the benefit of its directors and officers.



                                      II-3

<PAGE>   6



         Item 7.  EXEMPTION FROM REGISTRATION CLAIMED

                  Not applicable.


         Item 8.  EXHIBITS

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


         Item 9.  UNDERTAKINGS

                  1.       The undersigned 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 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) 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 15(d) of the Exchange Act that are
         incorporated by reference in the 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.

                  2. The undersigned Registrant hereby undertakes that, for
purposes of determining any liability under the Securities Act, each filing of
the Registrant's annual

                                      II-4

<PAGE>   7



report pursuant to Section 13(a) or 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 the
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 the initial bona fide offering thereof.

                  3. 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 Commission such
indemnification is against public policy as expressed in the Securities 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.




                                      II-5

<PAGE>   8



                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, 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 Town of Bedford, Massachusetts on August 5, 1999.


                                            SECURITY DYNAMICS TECHNOLOGIES, INC.



                                            By:  /s/ Charles R. Stuckey, Jr.
                                                ________________________________
                                                 Charles R. Stuckey, Jr.
                                                 Chairman of the Board and
                                                 Chief Executive Officer



                                POWER OF ATTORNEY

         We, the undersigned officers and directors of Security Dynamics
Technologies, Inc. hereby severally constitute Charles R. Stuckey, Jr., Arthur
W. Coviello, Jr. and Hal J. Leibowitz, and each of them singly, our true and
lawful attorneys with full power to them, and each of them singly, to sign for
us and in our names in the capacities indicated below, the Registration
Statement on Form S-8 filed herewith and any and all subsequent amendments to
said Registration Statement, and generally to do all such things in our names
and behalf in our capacities as officers and directors to enable Security
Dynamics Technologies, Inc. to comply with all requirements of the Securities
and Exchange Commission, hereby ratifying and confirming our signatures as they
may be signed by said attorneys, or any of them, to said Registration Statement
and any and all amendments thereto.





                                      II-6

<PAGE>   9



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


      SIGNATURE                        TITLE                         DATE

/s/ Charles R. Stuckey, Jr.    Chairman of the Board             August 5, 1999
---------------------------    of Directors and Chief
Charles R. Stuckey, Jr.        Executive Officer
                               (Principal Executive
                               Officer)

/s/ John F. Kennedy            Senior Vice President             August 5, 1999
---------------------------    and Chief Financial
John F. Kennedy                Officer (Principal
                               Financial and
                               Accounting Officer)

                               Director
---------------------------
D. James Bidzos


/s/ Arthur W. Coviello, Jr.    Director                          August 5, 1999
---------------------------
Arthur W. Coviello, Jr.


/s/ Richard L. Earnest         Director                          August 5, 1999
---------------------------
Richard L. Earnest


                               Director
---------------------------
Taher Elgamal


/s/ Joseph B. Lassiter, III    Director                          August 5, 1999
---------------------------
Joseph B. Lassiter, III


                               Director
---------------------------
George M. Middlemas


/s/ James K. Sims              Director                          August 5, 1999
---------------------------
James K. Sims


                                  II-7


<PAGE>   10

                                  EXHIBIT INDEX


 Exhibit
 Number                         Description
---------                       -----------

   4.1         Specimen Certificate of Common Stock, $.01 par value per share,
               of the Registrant is incorporated herein by reference to Exhibit
               4.1 to the Registrant's Registration Statement on Form S-1, as
               amended (File No. 33-85606)

    5          Opinion of Hale and Dorr LLP

  23.1         Consent of Hale and Dorr LLP (included in Exhibit 5)

  23.2         Consent of Deloitte & Touche LLP, independent auditors

   24          Power of Attorney (included in the signature pages of this
               Registration Statement)





                                      II-8
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{"company-data": {"conformed-name": "SECURITY DYNAMICS TECHNOLOGIES INC /DE/", "cik": "0000932064", "assigned-sic": "3577", "irs-number": "042916506", "state-of-incorporation": "DE", "fiscal-year-end": "1231"}, "filing-values": {"form-type": "S-8", "act": "33", "file-number": "333-84597", "film-number": "99678802"}, "business-address": {"street1": "36 CROSBY DRIVE", "city": "BEDFORD", "state": "MA", "zip": "01730", "phone": "6176877000"}, "mail-address": {"street1": "36 CROSBY DRIVE", "city": "BEDFORD", "state": "MA", "zip": "01730"}}
1
     As filed with the Securities and Exchange Commission on August 5, 1999

                                                   Registration No. 333-________

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

                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D. C. 20549

                                    FORM S-8

                          REGISTRATION STATEMENT UNDER
                           THE SECURITIES ACT OF 1933

                      SECURITY DYNAMICS TECHNOLOGIES, INC.
             (Exact Name of Registrant as Specified in Its Charter)

            DELAWARE                                         04-2916506
(State or Other Jurisdiction of                           (I.R.S. Employer
 Incorporation or Organization)                        Identification Number)

36 CROSBY DRIVE, BEDFORD, MASSACHUSETTS                        01730
(Address of Principal Executive Offices)                     (Zip Code)

                  1994 EMPLOYEE STOCK PURCHASE PLAN, AS AMENDED
                            (Full Title of the Plan)

                             ARTHUR W. COVIELLO, JR.
                      SECURITY DYNAMICS TECHNOLOGIES, INC.
                                 36 CROSBY DRIVE
                          BEDFORD, MASSACHUSETTS 01730
                     (Name and Address of Agent for Service)

                                 (781) 301-5000
          (Telephone Number, Including Area Code, of Agent for Service)
2

                         CALCULATION OF REGISTRATION FEE

==============    ==========      ===========      ==============   ============
                                   Proposed          Proposed
   Title of                       Maximum            Maximum
  Securities        Amount        Offering          Aggregate        Amount of
    to be           to be           Price            Offering       Registration
  Registered      Registered      Per Share           Price             Fee
 ------------      --------       ---------         ------------     ----------

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

Common Stock,      600,000        $17.5625(1)      $10,537,500(1)      $2,930
$.01 par value     shares
==============    ==========      ===========      ==============   ============

(1)      Estimated solely for the purpose of calculating the registration fee,
         and based on the average of the high and low prices of the Common Stock
         on the Nasdaq National Market on July 30, 1999 in accordance with Rules
         457(c) and 457(h) under the Securities Act of 1933.

================================================================================
3

PART I.  INFORMATION REQUIRED IN THE SECTION 10(a) PROSPECTUS

                  The information required by Part I is included in documents
sent or given to participants in the Registrant's 1994 Employee Stock Purchase
Plan, as amended, pursuant to Rule 428(b)(1) of the Securities Act of 1933, as
amended (the "Securities Act").

PART II.  INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

         Item 3.  INCORPORATION OF DOCUMENTS BY REFERENCE

                  The Registrant is subject to the informational and reporting
requirements of Sections 13(a), 14 and 15(d) of the Securities Exchange Act of
1934, as amended (the "Exchange Act"), and in accordance therewith files
reports, proxy statements and other information with the Securities and Exchange
Commission (the "Commission"). The following documents, which are on file with
the Commission, are incorporated in this Registration Statement by reference:

                  (1) The Registrant's latest annual report filed pursuant to
         Section 13(a) or 15(d) of the Exchange Act, or the latest prospectus
         filed pursuant to Rule 424(b) under the Securities Act that contains
         audited financial statements for the Registrant's latest fiscal year
         for which such statements have been filed.

                  (2) All other reports filed pursuant to Section 13(a) or 15(d)
         of the Exchange Act since the end of the fiscal year covered by the
         document referred to in (1) above.

                  (3) The description of the common stock of the Registrant,
         $.01 par value per share (the "Common Stock"), contained in a
         registration statement filed under the Exchange Act, including any
         amendment or report filed for the purpose of updating such description.

                  All documents subsequently filed 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 shares of Common Stock
offered hereby have been sold or which deregisters all shares of Common Stock
then remaining unsold, shall be deemed to be incorporated by reference herein
and to be part hereof from the date of the filing of such documents. Any
statement contained in a document incorporated or deemed to be incorporated by
reference herein shall be deemed to be modified or superseded for purposes of
this Registration Statement to the extent that 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.

                                      II-1
4

         Item 4.  DESCRIPTION OF SECURITIES

                  Not applicable.

         Item 5.  INTERESTS OF NAMED EXPERTS AND COUNSEL

                  The legality of the Common Stock being offered hereby will be
passed upon for the Company by Hale and Dorr LLP, Boston, Massachusetts. A
senior partner of Hale and Dorr LLP beneficially owns approximately 3,300 shares
of the Registrant's Common Stock.

         Item 6.  INDEMNIFICATION OF DIRECTORS AND OFFICERS

                  Article EIGHTH of the Registrant's Third Restated Certificate
of Incorporation, as amended (the "Restated Certificate of Incorporation"),
provides that no director of the Registrant shall be personally liable for any
monetary damages for any breach of fiduciary duty as a director, except to the
extent that the Delaware General Corporation Law prohibits the elimination or
limitation of liability of directors for breach of fiduciary duty.

                  Article NINTH of the Registrant's Restated Certificate of
Incorporation provides that a director or officer of the Registrant (a) shall be
indemnified by the Registrant against all expenses (including attorneys' fees),
judgments, fines and amounts paid in settlement incurred in connection with any
litigation or other legal proceeding (other than an action by or in the right of
the Registrant) brought against him by virtue of his position as a director or
officer of the Registrant if he acted in good faith and in a manner he
reasonably believed to be in, or not opposed to, the best interests of the
Registrant, and, with respect to any criminal action or proceeding, had no
reasonable cause to believe his conduct was unlawful and (b) shall be
indemnified by the Registrant against all expenses (including attorneys' fees)
and amounts paid in settlement incurred in connection with any action by or in
the right of the Registrant brought against him by virtue of his position as a
director or officer of the Registrant if he acted in good faith and in a manner
he reasonably believed to be in, or not opposed to, the best interests of the
Registrant, except that no indemnification shall be made with respect to any
matter as to which such person shall have been adjudged to be liable to the
Registrant, unless a court determines that, despite such adjudication but in
view of all of the circumstances, he is entitled to indemnification of such
expenses. Notwithstanding the foregoing, to the extent that a director or
officer has been successful, on the merits or otherwise, including, without
limitation, the dismissal of an action without prejudice, he is required to be
indemnified by the Registrant against all expenses (including attorneys' fees)
incurred in connection therewith. Expenses shall be advanced to a director or
officer at his request, provided that he undertakes to repay the amount advanced
if it is ultimately determined that he is not entitled to indemnification for
such expenses.

                  Indemnification is required to be made unless the Registrant
determines that the applicable standard of conduct required for indemnification
has not been met. In the event of a determination by the Registrant that the
director or officer did not meet

                                      II-2
5

the applicable standard of conduct required for indemnification, or if the
Registrant fails to make an indemnification payment within 60 days after such
payment is claimed by such person, such person is permitted to petition the
court to make an independent determination as to whether such person is entitled
to indemnification. As a condition precedent to the right of indemnification,
the director or officer must give the Registrant notice of the action for which
indemnity is sought and the Registrant has the right to participate in such
action or assume the defense thereof.

                  Article NINTH of the Registrant's Restated Certificate of
Incorporation further provides that the indemnification provided therein is not
exclusive, and provides that in the event that the Delaware General Corporation
Law is amended to expand the indemnification permitted to directors or officers
the Registrant must indemnify those persons to the fullest extent permitted by
such law as so amended.

                  Section 145 of the Delaware General Corporation Law, as
amended, provides that a corporation may indemnify any person who was or is a
party or is threatened to be made a party to any threatened, pending or
completed action, suit or proceeding, whether civil, criminal, administrative or
investigative (other than an action by or in the right of the corporation) by
reason of the fact that he is or was a director, officer, employee or agent of
the corporation, or is or was serving at the request of the corporation as a
director, officer, employee or agent of another corporation, partnership, joint
venture, trust or other enterprise, against expenses (including attorneys'
fees), judgments, fines and amounts paid in settlement actually and reasonably
incurred by him in connection with such action, suit or proceeding if he acted
in good faith and in a manner he reasonably believed to be in or not opposed to
the best interests of the corporation, and, with respect to any criminal action
or proceeding, had no reasonable cause to believe his conduct was unlawful.
Section 145 further provides that a corporation similarly may indemnify any
person who was or is a party or is threatened to be made a party to any
threatened, pending or completed action or suit by or in the right of the
corporation to procure a judgment in its favor by reason of the fact that he is
or was a director, officer, employee or agent of the corporation, or is or was
serving at the request of the corporation as a director, officer, employee or
agent of another corporation, partnership, joint venture, trust or other
enterprise, against expenses (including attorneys' fees) actually and reasonably
incurred by him in connection with the defense or settlement of such action or
suit if he acted in good faith and in a manner he reasonably believed to be in
or not opposed to the best interests of the corporation, except that no
indemnification shall be made in respect of any claim, issue or matter as to
which such person shall have been adjudged to be liable to the corporation
unless and only to the extent that the Delaware Court of Chancery or the court
in which such action or suit was brought shall determine upon application that,
despite an adjudication of liability but in view of all the circumstances of the
case, such person is fairly and reasonably entitled to indemnity for such
expenses which the Court of Chancery or such other court shall deem proper.

                  The Registrant has directors and officers liability insurance
for the benefit of its directors and officers.

                                      II-3
6

         Item 7.  EXEMPTION FROM REGISTRATION CLAIMED

                  Not applicable.

         Item 8.  EXHIBITS

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

         Item 9.  UNDERTAKINGS

                  1.       The undersigned 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 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) 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 15(d) of the Exchange Act that are
         incorporated by reference in the 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.

                  2. The undersigned Registrant hereby undertakes that, for
purposes of determining any liability under the Securities Act, each filing of
the Registrant's annual

                                      II-4
7

report pursuant to Section 13(a) or 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 the
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 the initial bona fide offering thereof.

                  3. 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 Commission such
indemnification is against public policy as expressed in the Securities 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.

                                      II-5
8

                                   SIGNATURES

         Pursuant to the requirements of the Securities Act, 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 Town of Bedford, Massachusetts on August 5, 1999.

                                            SECURITY DYNAMICS TECHNOLOGIES, INC.

                                            By:  /s/ Charles R. Stuckey, Jr.
                                                ________________________________
                                                 Charles R. Stuckey, Jr.
                                                 Chairman of the Board and
                                                 Chief Executive Officer

                                POWER OF ATTORNEY

         We, the undersigned officers and directors of Security Dynamics
Technologies, Inc. hereby severally constitute Charles R. Stuckey, Jr., Arthur
W. Coviello, Jr. and Hal J. Leibowitz, and each of them singly, our true and
lawful attorneys with full power to them, and each of them singly, to sign for
us and in our names in the capacities indicated below, the Registration
Statement on Form S-8 filed herewith and any and all subsequent amendments to
said Registration Statement, and generally to do all such things in our names
and behalf in our capacities as officers and directors to enable Security
Dynamics Technologies, Inc. to comply with all requirements of the Securities
and Exchange Commission, hereby ratifying and confirming our signatures as they
may be signed by said attorneys, or any of them, to said Registration Statement
and any and all amendments thereto.

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9

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

      SIGNATURE                        TITLE                         DATE

/s/ Charles R. Stuckey, Jr.    Chairman of the Board             August 5, 1999
---------------------------    of Directors and Chief
Charles R. Stuckey, Jr.        Executive Officer
                               (Principal Executive
                               Officer)

/s/ John F. Kennedy            Senior Vice President             August 5, 1999
---------------------------    and Chief Financial
John F. Kennedy                Officer (Principal
                               Financial and
                               Accounting Officer)

                               Director
---------------------------
D. James Bidzos

/s/ Arthur W. Coviello, Jr.    Director                          August 5, 1999
---------------------------
Arthur W. Coviello, Jr.

/s/ Richard L. Earnest         Director                          August 5, 1999
---------------------------
Richard L. Earnest

                               Director
---------------------------
Taher Elgamal

/s/ Joseph B. Lassiter, III    Director                          August 5, 1999
---------------------------
Joseph B. Lassiter, III

                               Director
---------------------------
George M. Middlemas

/s/ James K. Sims              Director                          August 5, 1999
---------------------------
James K. Sims

                                  II-7
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                                  EXHIBIT INDEX

 Exhibit
 Number                         Description
---------                       -----------

   4.1         Specimen Certificate of Common Stock, $.01 par value per share,
               of the Registrant is incorporated herein by reference to Exhibit
               4.1 to the Registrant's Registration Statement on Form S-1, as
               amended (File No. 33-85606)

    5          Opinion of Hale and Dorr LLP

  23.1         Consent of Hale and Dorr LLP (included in Exhibit 5)

  23.2         Consent of Deloitte & Touche LLP, independent auditors

   24          Power of Attorney (included in the signature pages of this
               Registration Statement)

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