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                                 UNITED STATES

                       SECURITIES AND EXCHANGE COMMISSION

                             WASHINGTON, D.C. 20549

                                  FORM 10-K/A

                                AMENDMENT NO. 1


       FOR ANNUAL AND TRANSITION REPORTS PURSUANT TO SECTIONS 13 OR 
                  15(d) OF THE SECURITIES EXCHANGE ACT OF 1934

       [X] ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
                              EXCHANGE ACT OF 1934

                   FOR THE FISCAL YEAR ENDED DECEMBER 31, 1997

                                       OR

          [ ] TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE
                         SECURITIES EXCHANGE ACT OF 1934

                           COMMISSION FILE NO. 0-25120

                      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 No.)

             20 CROSBY DRIVE
         BEDFORD, MASSACHUSETTS                           01730
         (Address of principal                         (Zip Code)
           executive offices)

        REGISTRANT'S TELEPHONE NUMBER, INCLUDING AREA CODE: (781) 687-7000

        ------------------------------------------------------------------

        SECURITIES REGISTERED PURSUANT TO SECTION 12(B) OF THE ACT: NONE

           SECURITIES REGISTERED PURSUANT TO SECTION 12(G) OF THE ACT:
                          COMMON STOCK, $.01 PAR VALUE
                                (Title of class)


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      Indicate by check mark whether the registrant: (1) has filed all reports
required to be filed by Section 13 or 15(d) of the Securities Exchange Act of
1934 during the preceding 12 months (or for such shorter period that the
registrant was required to file such reports), and (2) has been subject to such
filing requirements for the past 90 days. Yes  X    No 
                                              ---     ---

      Indicate by check mark if disclosure of delinquent filers pursuant to Item
405 of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any amendment to this
Form 10-K. [ ]

      The approximate aggregate market value of the common stock held by
non-affiliates of the registrant was $1,640,944,886 based on the last reported
sale price of the registrant's Common Stock on the Nasdaq National Market as of
the close of business on March 26, 1998. There were 40,857,538 shares of Common
Stock outstanding as of March 26, 1998.

                       DOCUMENTS INCORPORATED BY REFERENCE

                                                         PART OF FORM 10-K
                DOCUMENT                              INTO WHICH INCORPORATED
                --------                              -----------------------

      Portions of the Registrant's
      1997 Annual Report to Stockholders               Items 6, 7 & 8 of Part II

      Portions of the Registrant's Proxy                 Items 10, 11 & 12
      Statement for the 1998 Annual Meeting                 of Part III
      of Stockholders

   This Annual Report on Form 10-K contains forward-looking statements within
the meaning of Section 21E of the Securities Exchange Act of 1934, as amended.
For this purpose, any statements contained herein that are not statements of
historical fact may be deemed to be forward-looking statements. Without limiting
the foregoing, the words "believes," "anticipates," "plans," "expects" and
similar expressions are intended to identify forward-looking statements. The
important factors discussed under the caption "Certain Factors That May Affect
Further Operating Results" in the Company's 1997 Annual Report to Stockholders
and incorporated herein by reference, among others, could cause actual results
to differ materially from those indicated by forward-looking statements made
herein and presented elsewhere by management. Such forward-looking statements
represent management's current expectations and are inherently uncertain.
Investors are warned that actual results may differ from management's
expectations.


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     This Amendment No. 1 on Form 10-K/A amends and restates the signature page
of the Annual Report on Form 10-K filed with the Securities and Exchange
Commission on March 31, 1998 by Security Dynamics Technologies, Inc. (the
"Company"), for the purpose of correcting a typographical error which resulted
in the inadvertent omission of the conformed signature of a member of the
Company's Board of Directors.











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                                   SIGNATURES

      Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.


                              SECURITY DYNAMICS TECHNOLOGIES, INC.


                              By:/s/ CHARLES R. STUCKEY, JR.
                                 ---------------------------------
                                 Charles R. Stuckey, Jr.
                                 Chairman, President and Chief Executive Officer

Date:  March 31, 1998

      Pursuant to the requirements of the Securities Exchange Act of 1934, this
report has been signed below by the following persons on behalf of the
Registrant and in the capacities and on the dates indicated.

Signature                         Title                           Date


/s/ CHARLES R. STUCKEY, JR.       Chairman, President and        March 31, 1998
--------------------------        Chief Executive Officer      
Charles R. Stuckey, Jr.           (Principal Executive Officer)


/s/ MARIAN G. O'LEARY             Senior Vice President,         March 31, 1998
---------------------             Finance, Chief Financial 
Marian G. O'Leary                 Officer and Treasurer    
                                  (Principal Financial and 
                                  Accounting Officer)      
                                  

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


                                  Director                     
----------------------
Richard L. Earnest


/s/ JOSEPH B. LASSITER, III       Director                       March 31, 1998
---------------------------
Joseph B. Lassiter, III


/s/ GEORGE M. MIDDLEMAS           Director                       March 31, 1998
-----------------------
George M. Middlemas


/s/ SANFORD M. SHERIZEN           Director                       March 31, 1998
-----------------------
Sanford M. Sherizen


                                  Director                      
-----------------
James K. Sims


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                                   SIGNATURE


     Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this report to be signed on
its behalf by the undersigned, thereunto duly authorized.



                                          SECURITY DYNAMICS TECHNOLOGIES, INC.



                                          By:  /s/ Arthur W. Coviello, Jr.
                                               ---------------------------------
                                               Arthur W. Coviello, Jr.
                                               Executive Vice President and 
                                               Chief Operating Officer

                                             
Date: April 1, 1998





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