1
                                                                     EXHIBIT 5.1



                                                   Suzanne Sawochka Hooper, Esq.
                                                   Direct: (650) 843-5180
                                                   Internet:hooperss@cooley.com

June 18, 2001


Kosan Biosciences Incorporated
3832 Bay Center Place
Hayward, CA  94545

Ladies and Gentlemen:


You have requested our opinion with respect to certain matters in connection
with the filing by Kosan Biosciences Incorporated (the "Company") of a
Registration Statement on Form S-8 (the "Registration Statement") with the
Securities and Exchange Commission covering the offering of an aggregate of
375,000 shares of the Company's common stock, $.001 par value (the "Shares"),
pursuant to the Company's 1996 Stock Option Plan, as amended (the "Plan").


In connection with this opinion, we have examined the Registration Statement and
related Prospectus, your Amended and Restated Certificate of Incorporation and
Bylaws and such other documents, records, certificates, memoranda and other
instruments as we deem necessary as a basis for this opinion. We have assumed
the genuineness and authenticity of all documents submitted to us as originals,
the conformity to originals of all documents submitted to us as copies thereof
and the due execution and delivery of all documents where due execution and
delivery are a prerequisite to the effectiveness thereof.


On the basis of the foregoing, and in reliance thereon, we are of the opinion
that the Shares, when sold and issued in accordance with the Plan, the
Registration Statement and the related Prospectus, will be validly issued, fully
paid and nonassessable (except as to shares issued pursuant to certain deferred
payment arrangements, which will be fully paid and nonassessable when such
deferred payments are made in full).


We consent to the filing of this opinion as an exhibit to the Registration
Statement.


Very truly yours,


Cooley Godward LLP



By: /s/ SUZANNE SAWOCHKA HOOPER
   -------------------------------
        Suzanne Sawochka Hooper