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                                                                EXHIBIT 18

August 13, 1996

The Board of Directors
Security Dynamics Technologies, Inc.
20 Crosby Drive
Bedford, Massachusetts

Dear Sirs:

At your request, we have read the description included in your Quarterly Report
on Form 10-Q to the Securities and Exchange Commission for the quarter ended
June 30, 1996, of the facts relating to Security Dynamics Technologies, Inc.'s
(the "Company") change, effective April 1, 1996, in its accounting policy for
option grants to purchase the Company's common stock requiring stockholder
approval to measure compensation expense on the approval date. We believe, on
the basis of the facts so set forth and other information furnished to us by
appropriate officials of the Company, that the accounting change described in
your Form 10-Q is to an alternative accounting principle that is preferable
under the circumstances.

We have not audited any consolidated financial statements of the Company and its
consolidated subsidiaries as of any date for any period subsequent to December
31, 1995. Therefore, we are unable to express an opinion on the facts set forth
in the above-mentioned Form 10-Q, on the related information furnished to us by
officials of the Company, or on the financial position, results of operations,
or cash flows of the Company and its consolidated subsidiaries as of any date or
for any period subsequent to December 31, 1995.

Yours truly,

/s/  DELOITTE & TOUCHE LLP

Boston, Massachusetts