EXHIBIT 10.21

                              MYCOGEN CORPORATION
                        COMMON STOCK PURCHASE AGREEMENT




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                              MYCOGEN CORPORATION

                        COMMON STOCK PURCHASE AGREEMENT


    This COMMON STOCK PURCHASE AGREEMENT ("Agreement") is made as of December
13, 1995 by and between Mycogen Corporation (the "Company"), a California
corporation, and Pioneer Overseas Corporation, an Iowa corporation (the
"Purchaser").

    IN CONSIDERATION of the mutual covenants contained in this Agreement, the
Company and the Purchaser agree as follows:

    SECTION 1. AGREEMENT TO SELL AND PURCHASE THE COMMON STOCK. At the Closing
(as defined in Section 2), the Company shall sell to the Purchaser, and the
Purchaser shall purchase from the Company, upon the terms and conditions
hereinafter set forth, Three Million (3,000,000) shares (the "Shares") of the
Company's Common Stock (the "Common Stock") for a purchase price per share of
Ten Dollars ($10.00), which results in an aggregate purchase price for the
Shares of Thirty Million Dollars ($30,000,000).
 
    SECTION 2. DELIVERY OF THE COMMON STOCK AT THE CLOSING. The completion of
the purchase and sale of the Shares (the "Closing") shall occur at the principal
offices of the Company at 5501 Oberlin Drive, San Diego, California 92121
(telephone number 619/453-8030; facsimile number 619/453-5494) at 10:00 a.m. on
December 13, 1995 (the "Closing Date"), or such later date as the Company and
the Purchaser may agree, subject to the satisfaction (or waiver) of the
conditions hereinafter set forth. At the Closing, the Purchaser shall make
payment of the full purchase price for the Shares by wire transfer of same-day
funds as directed by the Company in writing. At the Closing, the Company shall
deliver to the Purchaser one or more stock certificates registered in the name
of the Purchaser representing the Shares.

    SECTION 3. REPRESENTATIONS AND WARRANTIES OF THE COMPANY. The Company
represents and warrants to the Purchaser as of the Closing Date as follows:

    3.1.  ORGANIZATION.  The Company is a corporation duly organized and
validly existing and in good standing under the laws of the State of California.
The Company has all requisite corporate power and authority to own, lease and
operate its properties and assets, and to carry on its business as presently
conducted and as proposed to be conducted and to execute and deliver this
Agreement and to consummate the transaction contemplated herein.  The Company is
qualified to do business as a foreign corporation in each jurisdiction in which
the failure to so qualify would have a material adverse effect on the condition
(financial or otherwise), assets, business or results of operations of the
Company and its Subsidiaries (as hereinafter defined) taken as a whole (a
"Material Adverse Effect").
 
     3.2.  SUBSIDIARIES.  All of the Company's subsidiaries (the "Subsidiaries")
are listed on Exhibit 21 to the Company's Annual Report on Form 10-K for the
Year Ended August 31, 1995 (the "Form 10-K").  Each of the Company's
Subsidiaries is a corporation duly incorporated, validly existing and in good
standing under the laws of the jurisdiction of its incorporation, has full

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corporate power and authority to own and lease its properties, and to carry on
its business as presently conducted, is duly qualified, registered or licensed
as a foreign corporation to do business and is in good standing in each
jurisdiction in which the ownership or leasing of its properties or the
character of its present operations make such qualification, registration or
licensing necessary, except where the failure so to qualify or be in good
standing would not have a Material Adverse Effect.
 
     3.3.  NO BREACH.  The execution and delivery of this Agreement by the
Company does not, and the issuance of the Shares of the Company will not, (i)
violate or conflict with the Certificate or Articles of Incorporation or Bylaws
of the Company, (ii) constitute a breach or default (or an event that with
notice or lapse of time or both would become a breach or default) of, or give
rise to any lien, third-party right of termination, cancellation, modification
or acceleration under, any agreement, understanding or undertaking to which the
Company is a party, except where such breach, default, lien, third-party right,
cancellation, modification or acceleration would not have a Material Adverse
Effect, or (iii) subject to obtaining the approvals and making the filings
described in Section 3.6 hereof, constitute a violation of any statute, law,
ordinance, rule, regulation, judgment, decree, order or writ of any judicial,
arbitral, public, or governmental authority having jurisdiction over the Company
or any of its Subsidiaries or any of their respective properties or assets
except as would not have a Material Adverse Effect.
 
     3.4.  ISSUANCE AND DELIVERY OF THE SHARES.  The Shares, when issued in
compliance with the provisions of this Agreement, will be validly issued, fully
paid and nonassessable.  The issuance and delivery of the Shares is not subject
to preemptive or any other similar rights of the stockholders of the Company or
any liens or encumbrances.
 
     3.5.  SEC DOCUMENTS; FINANCIAL STATEMENTS.  The Company has filed in a
timely manner all documents that the Company was required to file with the
Securities and Exchange Commission (the "SEC") under Sections 13, 14(a) and
15(d) of the Securities Exchange Act of 1934, as amended (the "Exchange Act"),
during the twelve (12) months preceding the date of this Agreement.  As of their
respective filing dates, all documents filed by the Company with the SEC (the
"SEC Documents") complied in all material respects with the requirements of the
Exchange Act or the Securities Act of 1933, as amended (the "Securities Act"),
as applicable.  None of the SEC Documents, including the financial statements or
schedules included or incorporated therein, as of their respective dates
contained any untrue statement of a material fact or omitted to state a material
fact required to be stated therein or necessary to make the statements made
therein, in the light of the circumstances under which they were made, not
misleading.  The financial statements of the Company included in the SEC
Documents (the "Financial Statements") comply as to form in all material
respects with applicable accounting requirements and with the published rules
and regulations of the SEC with respect thereto.  The Financial Statements have
been prepared in accordance with generally accepted accounting principles
consistently applied and fairly present the consolidated financial position of
the company and any Subsidiaries at the dates thereof and the consolidated
results of their operations and consolidated cash flows for the periods then
ended (subject, in the case of unaudited statements, to normal, recurring
adjustments).  As of the Closing 

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Date, the Company is in compliance in all material respects with the
requirements of the Exchange Act and the Securities Act.
 
     3.6.  GOVERNMENTAL CONSENTS.  No consent, approval, order or authorization
of, or registration, qualification, designation, declaration or filing with, any
federal, state, or local governmental authority on the part of the Company is
required in connection with the execution and delivery of the Agreement, or the
consummation of the transactions contemplated by this Agreement except for (a)
the expiration or early termination of the waiting period under the Hart-Scott-
Rodino Antitrust Improvements Act of 1976, (b) compliance with the securities
and blue sky laws in the states in which Shares are offered and/or sold, which
compliance will be effected in accordance with such laws, and (c) the filing of
The NASDAQ National Market Notification Form with The NASDAQ National Market and
Form 10-C with the SEC.
 
     3.7.  NO MATERIAL ADVERSE CHANGE.  Except as otherwise disclosed herein or
in the Company's most recent filed Form 10-K (a copy of which has been provided
to the Purchaser), since August 31, 1995, there have not been any changes in the
assets, liabilities, financial condition, business prospects or operations of
the Company from that reflected in the Financial Statements except changes in
the ordinary course of business and changes which would not have, either
individually or in the aggregate, a Material Adverse Effect or have a material
adverse effect on the ability of the Company to perform its obligations under
this Agreement.
 
     3.8.  AUTHORIZED CAPITAL STOCK.  The authorized capital stock of the
Company consists of 40,000,000 shares of Common Stock $.001 par value, and
5,000,000 shares of serial preferred stock, $.001 par value, of which 19,481,106
shares of Common Stock and 3,100 shares of Series A Preferred Stock are
outstanding prior to the issuance of the Shares.  Except as described in the
Company's Form 10-K, there are no outstanding options, warrants, puts, calls,
commitments, convertible or exchangeable securities or similar rights requiring
or providing for the issuance of new or additional equity interests in the
Company.  All such outstanding shares are duly authorized, validly issues and
fully paid and nonassessable.  There are no preemptive or other similar rights
available to the existing holders of the capital stock of the Company.  There
are no voting trusts or other agreements or understandings to which the Company
is a party with respect to the voting of capital stock of the Company.
 
     3.9.  LITIGATION.  There are no actions, suits, proceedings or
investigations pending or, to the best of the Company's knowledge, threatened
against the Company or any of its properties before or by any court or
arbitrator or any governmental body, agency or official in which there is a
reasonable likelihood (in the judgment of the Company) of an adverse decision
that (a) could have a Material Adverse Effect or (b) could impair the ability of
the Company to perform in any material respect its obligations under this
Agreement.
 
     3.10.  COMPLIANCE WITH LAW.  The Company holds all licenses, franchises,
certificates, consents, permits and authorizations from all governmental
authorities necessary for the lawful conduct of its business, except where the
failure to hold any of the foregoing would not have a Material Adverse Effect.
To the Company's knowledge, the Company has not violated, and is not 

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in violation of, any such licenses, franchises, certificates, consents, permits
or authorizations or any applicable statutes, laws, ordinances, rules and
regulations (including, without limitation, any of the foregoing related to
occupational safety, storage, disposal, discharge into the environment of
hazardous wastes, environmental protection, conservation, unfair competition,
labor practices or corrupt practices) of any governmental authorities, except
where such violations do not, and insofar as reasonably can be foreseen, will
not have a Material Adverse Effect, and the Company has not received any notice
from a governmental or regulatory authority within three years of the date
hereof of any such violation.
 
     3.11.  USE OF PROCEEDS.  The Company will apply the net proceeds from the
sale of the Shares to redeem shares of its Series A Preferred Stock and for
general corporate purposes.
 
     3.12.  BROKERS AND FINDERS.  Neither the Company, nor any officer, director
or employee of the company has employed any broker or finder or incurred any
liability for any brokerage fees, commissions or finder's fees in connection
with the transactions contemplated herein.
 
     SECTION 4. REPRESENTATIONS, WARRANTIES AND COVENANTS OF THE PURCHASER. The
Purchaser represents and warrants to the Company as follows:
 
     4.1.  ACCREDITED INVESTOR.  The Purchaser is an "accredited investor"
within the meaning of Rule 501(a) of the Securities Act.
 
     4.2.  INVESTMENT REPRESENTATIONS.  The Purchaser is aware that the Shares
have not been registered under the Securities Act or any applicable state
securities laws, and agrees that the Shares will not be offered or sold in the
absence of registration under the Securities Act and any applicable state
securities laws or an exemption from the registration requirements of the
Securities Act and any applicable state securities laws.  The Purchaser will not
transfer the Shares in violation of the provisions of any applicable federal or
state securities laws.  In this connection, the Purchaser represents that it is
familiar with SEC Rule 144 promulgated pursuant to the Securities Act ("Rule
144"), as presently in effect, and understands the resale limitations imposed
thereby and by the Securities Act.

     The Purchaser understands that the offering and sale of the Shares is
intended to be exempt from registration under the Securities Act, by virtue of
Section 4(2) and/or Section 4(6) of the Securities Act and the provisions of
Regulation D promulgated thereunder, based, in part, upon the representations,
warranties and agreements contained in this Agreement and the Company may rely
on such representations, warranties and agreements in connection therewith.

     The Purchaser is acquiring the Shares for its own account and for
investment, and not with a view to the distribution thereof or with any present
intention of distributing or selling any of the Shares except in compliance with
the Securities Act. The Purchaser represents that by reason of its business and
financial experience, and the business and financial experience of those
persons, if any, retained by it to advise it with respect to its investment in
the Shares, such Purchaser together with such advisors have knowledge,
sophistication and experience in business and financial matters 

                                      112

 
as to be capable of evaluating the merits and risk of the prospective
investment. The Purchaser's financial condition and investments are such that it
is in a financial position to hold the Shares for an indefinite period of time
and to bear the economic risk of, and withstand a complete loss of, its
investment in the Shares.

     4.3.  AUTHORITY.  The Purchaser has full right, power, authority and
capacity to enter into this Agreement and to consummate the transactions
contemplated hereby.  Upon the execution and delivery of this Agreement by the
Purchaser and by the Company, this Agreement shall constitute a valid and
binding obligation of the Purchaser, enforceable in accordance with its terms,
except as enforceability may be limited by applicable bankruptcy, insolvency,
reorganization, moratorium or similar laws affecting creditors' and contracting
parties' rights generally and except as enforceability may be subject to general
principles of equity (regardless of whether such enforceability is considered in
a proceeding in equity or at law).
 
     4.4.  PURCHASER REVIEW.  The Purchaser has carefully examined the SEC
Documents.  The Purchaser acknowledges that the Company has made available to
the Purchaser all documents and information that it has requested relating to
the Company and has provided answers to all of its questions concerning the
Company and the Shares.  In evaluating the suitability of the acquisition of the
Shares hereunder, the Purchaser has not relied upon any representations or other
information (whether oral or written) other than as set forth in the SEC
Documents or as contained herein.
 
     SECTION 5. SURVIVAL OF REPRESENTATIONS, WARRANTIES AND AGREEMENTS. Notwith-
standing any investigation made by any party to this Agreement, all covenants,
agreements, representations and warranties made by the Company and the Purchaser
herein shall survive the execution of this Agreement, the delivery to the
Purchaser of the Shares being purchased and the payment therefor.

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     SECTION 6.  RESTRICTIONS ON TRANSFERABILITY OF SHARES, COMPLIANCE WITH
SECURITIES ACT.
 
     6.1.  RESTRICTIONS ON TRANSFERABILITY.  The Shares shall not be
transferable in the absence of an effective registration statement under the
Securities Act or an exemption therefrom or in the absence of compliance with
any term of this Agreement.  In the absence of an effective registration
statement under the Securities Act, neither the Shares nor any interest therein
shall be sold, transferred, assigned or otherwise disposed of, unless the
Company shall have previously received an opinion of counsel knowledgeable in
federal securities law, in form and substance reasonably satisfactory to the
Company and accompanied by such supporting documents as the Company may
reasonably request, to the effect that registration under the Securities Act is
not required in connection with such disposition.  The Company shall be entitled
to give stop transfer instructions to its transfer agent with respect to the
Shares in order to enforce the foregoing restrictions.
 
     6.2.  RESTRICTIVE LEGEND.  The certificate or certificates representing the
Shares shall bear the following legend restricting transfer:

     "THE SHARES REPRESENTED BY THIS CERTIFICATE HAVE BEEN ACQUIRED FOR
     INVESTMENT AND HAVE NOT BEEN REGISTERED UNDER THE SECURITIES ACT OF 1933,
     AS AMENDED. SUCH SHARES MAY NOT BE SOLD OR TRANSFERRED IN THE ABSENCE OF
     SUCH REGISTRATION OR AN EXEMPTION FROM REGISTRATION UNDER THE SECURITIES
     ACT, PROVIDED THAT AN OPINION OF COUNSEL REASONABLY SATISFACTORY IN FORM
     AND SUBSTANCE IS FURNISHED TO THE COMPANY THAT AN EXEMPTION FROM THE
     REGISTRATION REQUIREMENTS OF THE SECURITIES ACT IS AVAILABLE."

The certificate shall also include any legend required by any applicable state
securities law.

     SECTION 7. CONDITIONS TO OBLIGATIONS OF PURCHASER. The obligations of the
Purchaser to purchase the Shares set forth on the signature page hereof at the
Closing is subject to the fulfillment on or prior to the Closing Date of the
following conditions, any or all of which may be waived at the option of the
Purchaser:
 
     7.1.  REPRESENTATIONS AND WARRANTIES CORRECT.  The representations and
warranties made by the Company in Section 3 hereof shall be true and correct in
all material respects when made, and shall be true and correct in all material
respects on the Closing Date with the same force and effect as if they had been
made on and as of said date.

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     7.2.  COVENANTS.  All covenants, agreements and conditions contained in
this Agreement to be performed by the Company on or prior to the Closing Date
shall have been performed or complied with in all material respects.
 
     7.3.  NO PROHIBITION.  There shall not then be in effect any order
enjoining or restraining the transactions contemplated by this Agreement or any
law, rule or regulation prohibiting or restricting such transactions, or
requiring any consent or approval of any person which shall not have been
obtained (except as otherwise provided in this Agreement).
 
     7.4.  COMPLIANCE CERTIFICATE.  The Company shall have delivered to the
Purchaser a certificate executed on behalf of the Company by its Chief Operating
Officer and dated the Closing Date, certifying to the fulfillment of the
conditions specified in Sections 7.1 and 7.2.
 
     7.5.  COMPLIANCE WITH SECURITIES LAWS.  The offering, issuance and sale of
the Shares under this Agreement shall have complied with all applicable
requirements of federal securities laws and the Purchaser shall have received
evidence, if any, of such compliance in form and substance satisfactory to the
Purchaser.
 
     7.6.  REGISTRATION RIGHTS AGREEMENT.  The Company shall have executed and
delivered to the Purchaser the Registration Rights Agreement substantially in
the form attached hereto as Exhibit "A" (the "Registration Rights Agreement").
 
     7.7.  COLLABORATION AGREEMENT.  The Company shall have executed and deliver
to the Purchaser the Collaboration Agreement substantially in the form attached
hereto as Exhibit "B" (the "Collaboration Agreement").
 
     7.8.  PROCEEDINGS AND DOCUMENTS.  All corporate and other proceedings
contemplated by this Agreement shall be satisfactory to the Purchaser and such
Purchaser's counsel, and the Purchaser and such Purchaser's counsel shall have
received all such counterpart originals or certified or other copies of such
documents as the Purchaser or such Purchaser's counsel may reasonably request.
 
     SECTION 8. CONDITIONS TO OBLIGATIONS OF COMPANY. The Company's obligation
to issue and sell the Shares to the Purchaser at the Closing is subject to the
fulfillment on or prior to the Closing Date of the following conditions, any or
all of which may be waived at the option of the Company:
 
     8.1.  REPRESENTATIONS AND WARRANTIES CORRECT.  The representations and
warranties made by the Purchaser in Section 4 hereof shall be true and correct
in all material respects when made, and shall be true and correct in all
material respects on the Closing Date with the same force and effect as if they
had been made on and as of said date.

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     8.2.  COVENANTS.  All covenants, agreements and conditions contained in
this Agreement to be performed by the Purchaser on or prior to the Closing Date
shall have been performed or complied with in all material respects.
 
     8.3.  NO PROHIBITION.  There shall not then be in effect any order
enjoining or restraining the transactions contemplated by this Agreement, or any
law, rule or regulation prohibiting or restricting such transactions, or
requiring any consent or approval of any person which shall not have been
obtained (except as otherwise provided in this Agreement).
 
     8.4.  COMPLIANCE CERTIFICATE.  The Purchaser shall have delivered to the
Company a certificate executed on behalf of the Purchaser by an authorized
officer thereof and dated the Closing Date, certifying to the fulfillment of the
conditions specified in Sections 8.1 and 8.2.
 
     8.5.  COLLABORATION AGREEMENT.  The Purchaser shall have executed and
delivered to the Company the Collaboration Agreement.
 
     8.6.  PROCEEDINGS AND DOCUMENTS.  All corporate and other proceedings
contemplated by this Agreement shall be satisfactory to the Company and
Company's counsel, and the Company and Company's counsel shall have received all
such counterpart originals or certified or other copies of such documents as the
Company or Company's counsel may reasonably request.
 
     SECTION 9. NOTICES. All notices, requests, consents and other
communications hereunder shall be in writing and shall be deemed given when sent
both by facsimile (unless the addressee has not provided a valid facsimile
number for such purpose) and either first class mail, postage prepaid, or next-
day delivery service:
 
          (a) if to Company, to Mycogen Corporation, 5501 Oberlin Drive, San
Diego, California 92121, Attention:  Carlton J. Eibl, President and Chief
Operating Officer, facsimile number 619/453-0142 with a copy to Page, Polin,
Busch & Boatwright, 350 West Ash Street, Suite 900, San Diego, California 92101-
3436, Attention:  Steven G. Rowles, Esq., facsimile number 619/231-1996, or to
such other person at such other place as the Company shall designate to the
Purchaser in writing;
 
          (b) if to the Purchaser, to Pioneer Hi-Bred International, Inc., 700
Capital Square, 400 Locust Street, Des Moines, Iowa 50309, Attention:  Daniel
Cornelison, facsimile number 515/248-4844, or at such other facsimile number and
address as may have been furnished to the Company in writing; or
 
          (c) if to transferee or transferees of the Purchaser, at such
facsimile number and address as shall have been furnished by such transferee or
transferees to the Company in writing.
 
     SECTION 10. ENTIRE AGREEMENT. This Agreement embodies the entire agreement
and understanding between the parties hereto with respect to the subject matter
hereof and supersedes all prior oral or written agreements and understandings
relating to the subject matter hereof. No 

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statement, representation, warranty, covenant or agreement of any kind not
expressly set forth in this Agreement shall affect, or be used to interpret,
change or restrict, the express terms and provisions of this Agreement.
 
     SECTION 11. AMENDMENTS. This Agreement may not be modified or amended
except pursuant to an instrument in writing signed by the Company and by the
Purchaser.
 
     SECTION 12. HEADINGS. The headings of the various sections of this
Agreement have been inserted for convenience of reference only and shall not be
deemed to be part of this Agreement.
 
     SECTION 13. SEVERABILITY. In case any provision contained in this Agreement
should be invalid, illegal or unenforceable in any respect, the validity,
legality and enforceability of the remaining provisions contained herein shall
not in any way be affected or impaired thereby.
 
     SECTION 14. GOVERNING LAW. This Agreement shall be governed by and
construed in accordance with the laws of the State of California (without regard
to conflict of law principles) and the United States of America.
 
     SECTION 15. COUNTERPARTS. This Agreement may be executed in two or more
counterparts, each of which shall constitute an original, but all of which, when
taken together, shall constitute but one instrument, and shall become effective
when one or more counterparts have been signed by each party hereto and
delivered to the other party.
 
     SECTION 16. EXPENSES. Each of the parties shall pay its own fees and
expenses (including the fees of any attorneys, accountants, appraisers or others
engaged by such party) in connection with this Agreement and the transaction
contemplated hereby whether or not the transactions contemplated hereby are
consummated.
 
     SECTION 17. PUBLICITY. Purchaser shall not issue any press releases or
otherwise make any public statement with respect to the transactions
contemplated by this Agreement without the prior written consent of the Company,
except as may be required by applicable law or regulation.

                 [REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

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     IN WITNESS WHEREOF, the parties hereto have caused this Agreement to be
executed by their duly authorized representatives the day and year first above
written.

                                MYCOGEN CORPORATION, a California
                                corporation

                                By:   /s/ Carlton J. Eibl
                                      -------------------------------------
                                      Carlton J. Eibl
                                      President and Chief Operating Officer

                                PIONEER HI-BRED INTERNATIONAL, INC.,
                                an Iowa corporation


                                By:   /s/ Chuck Johnson
                                      -------------------------------------
                                      Chuck Johnson
                                      President and Chief Operating Officer



              [SIGNATURE PAGE TO COMMON STOCK PURCHASE AGREEMENT]




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                                  EXHIBIT "A"

                         REGISTRATION RIGHTS AGREEMENT





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                                  EXHIBIT "B"

                            COLLABORATION AGREEMENT




                                      120