Exhibit 5.1

                                December 3, 1999

Adaptec, Inc.
691 South Milpitas Boulevard
Milpitas, California 95035

         RE:  REGISTRATION STATEMENT ON FORM S-8

Ladies and Gentlemen:

         We have examined the Post-Effective Amendment No. 1 to Registration
Statement on Form S-8 to be filed by Adaptec, Inc., a Delaware Corporation (the
"Company"), with the Securities and Exchange Commission on or around December 3,
1999 (the "Registration Statement") in connection with the registration under
the Securities Act of 1933, as amended, of 4,490,166 shares of the Company's
Common Stock (the "Shares") reserved for issuance under the Company's 1999 Stock
Plan (the "Plan"). As your legal counsel, we have examined the proceedings taken
and are familiar with the proceedings proposed to be taken by you in connection
with the issuance and sale of the Shares under the Plan.

         It is our opinion that, upon completion of the proceedings being taken
or contemplated by us to be taken prior to the issuance and sale of the Shares
pursuant to the Plan, and upon completion of the proceedings being taken in
order to permit such transaction to be carried out in accordance with the
securities laws of the various states where required, the Shares, when issued
and sold in the manner referred to in the Plan and the Registration Statement,
will be legally and validly issued, fully-paid and non-assessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement, and further consent to the use of our name wherever appearing in the
Registration Statement, including the prospectus constituting a part thereof,
and any amendments thereto.

                                           Very truly yours,

                                           WILSON SONSINI GOODRICH & ROSATI
                                           Professional Corporation

                                           /s/ Wilson Sonsini Goodrich & Rosati

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