<PAGE>
 
                                  EXHIBIT 10.3



                                     A-1.
<PAGE>
 
                              MYCOGEN CORPORATION
                       1995 EMPLOYEE STOCK PURCHASE PLAN
                       ---------------------------------

I.     PURPOSE OF THE PLAN

       This Employee Stock Purchase Plan is intended to promote the interests of
       MYCOGEN CORPORATION by providing eligible employees with the opportunity
       to acquire a proprietary interest in the Corporation through
       participation in a payroll-deduction based employee stock purchase plan.

       Capitalized terms herein shall have the meanings assigned to such terms
       in the attached Appendix.

II.    ADMINISTRATION OF THE PLAN

       The Compensation Committee of the Board in its capacity as Plan
       Administrator shall have full authority to interpret and construe any
       provision of the Plan and to adopt such rules and regulations for proper
       administration of the Plan as it may deem necessary or appropriate.
       Decisions of the Plan Administrator shall be final and binding on all
       parties having an interest in the Plan.

III.   STOCK SUBJECT TO PLAN

       A. The stock purchasable under the Plan shall be shares of authorized but
          unissued or reacquired Common Stock, including shares of Common Stock
          purchased on the open market.  The maximum number of shares of Common
          Stock which may be issued over the term of the Plan shall not exceed
          250,000 shares.  Such authorized share reserve is comprised of (i) the
          estimated number of shares (184,000 shares) which will remain
          available for issuance under the Predecessor Plan following the
          November 30, 1995 purchase date thereunder plus (ii) an additional
          increase of approximately 66,000 shares.

       B. Should any change be made to the Common Stock by reason of any stock
          split, stock dividend, recapitalization, combination of shares,
          exchange of shares or other change affecting the outstanding Common
          Stock as a class without the Corporation's receipt of consideration,
          appropriate adjustments shall be made to (i) the maximum number and
          class of securities issuable under the Plan, (ii) the maximum number
          and class of securities purchasable per Participant on any one
          Purchase Date and (iii) the number and class of securities and the
          price per share in effect under each outstanding purchase right in
          order to prevent the dilution or enlargement of benefits thereunder.

IV.    PURCHASE PERIODS


                                     A-2.
<PAGE>
 
       A. Shares of Common Stock shall be offered for purchase under the Plan
          through a series of successive Purchase Periods until such time as (i)
          the maximum number of shares of Common Stock available for issuance
          under the Plan shall have been purchased or (ii) the Plan shall have
          been sooner terminated.

       B. Each Purchase Period shall be of six (6) months duration.  The initial
          Purchase Period shall commence on the Effective Date and shall end on
          the last business day in May 1996.  Subsequent Purchase Periods shall
          run from the first business day in June to the last business day in
          November each year and from the first business day of December each
          year to the last business day of May in the succeeding year.

       C. Under no circumstances shall any shares of Common Stock be issued
          under the Purchase Plan until such time as (i) the Plan shall have
          been approved by the Corporation's stockholders and (ii) the
          Corporation shall have complied with all applicable requirements of
          the Securities Act, all applicable listing requirements of any
          securities exchange (or the Nasdaq National Market if applicable) on
          which shares of the Common Stock are listed for trading and all other
          applicable statutory and regulatory requirements.

V.     ELIGIBILITY

       A. Each Eligible Employee may commence participation in the Plan on the
          start date of any Purchase Period coincident with or subsequent to his
          or her completion of one month of continuous service with the
          Corporation or any Corporate Affiliate, provided he or she remains an
          Eligible Employee on such start date.

       B. To participate in the Plan for a particular Purchase Period, the
          Eligible Employee must complete the enrollment forms prescribed by the
          Plan Administrator (including a stock purchase agreement and a payroll
          deduction authorization form) and file such forms with the Plan
          Administrator (or its designate) prior to the start date of that
          Purchase Period.

VI.    PAYROLL DEDUCTIONS


                                     A-3.
<PAGE>
 
       A. The payroll deduction authorized by the Participant for purposes of
          acquiring shares of Common Stock under the Plan may be any multiple of
          one percent (1%) of the Base Salary paid to the Participant during the
          Purchase Period, up to a maximum of fifteen percent (15%).  The
          deduction rate so authorized shall continue in effect from Purchase
          Period to Purchase Period.  However, the Participant may, prior to the
          commencement of any new Purchase Period, increase or decrease the rate
          of his or her payroll deduction by filing the appropriate form with
          the Plan Administrator prior to the start date of that Purchase
          Period.  The new rate (which may not exceed the fifteen percent (15%)
          maximum) shall become effective as of the start date of the first
          Purchase Period following the filing of such form.

      B.  Payroll deductions shall begin on the first pay day following the
          start date of the Purchase Period and shall (unless sooner terminated
          by the Participant) continue through the pay day ending with or
          immediately prior to the last day of that Purchase Period. The amounts
          so collected shall be credited to the Participant's book account under
          the Plan, but no interest shall be paid on the balance from time to
          time outstanding in such account. The amounts collected from the
          Participant shall not be held in any segregated account or trust fund
          and may be commingled with the general assets of the Corporation and
          used for general corporate purposes.

      C.  Payroll deductions shall automatically cease upon the termination of
          the Participant's purchase right in accordance with the provisions of
          the Plan.

      D.  The Participant's acquisition of Common Stock under the Plan on any
          Purchase Date shall neither limit nor require the Participant's
          acquisition of Common Stock on any subsequent Purchase Date.

VII.  PURCHASE RIGHTS

      A.  GRANT OF PURCHASE RIGHT.  A Participant shall be granted a separate
          -----------------------                                            
          purchase right for each Purchase Period in which he or she
          participates.  The purchase right shall be granted on the start date
          of the Purchase Period and shall provide the Participant with the
          right to purchase shares of Common Stock upon the terms set forth
          below.  The Participant shall execute a stock purchase agreement
          embodying such terms and such other provisions (not inconsistent with
          the Plan) as the Plan Administrator may deem advisable.

          Under no circumstances shall purchase rights be granted under the
          Plan to any Eligible Employee if such individual would, immediately
          after the grant, own (within the meaning of Code Section 424(d)) or
          hold outstanding options or other rights to purchase, stock possessing
          five percent (5%) or


                                     A-4.
<PAGE>
 
          more of the total combined voting power or value of all classes of
          stock of the Corporation or any Corporate Affiliate.

      B.  EXERCISE OF THE PURCHASE RIGHT.  Each purchase right shall be
          ------------------------------                               
          automatically exercised on the last day of the Purchase Period by
          applying the Participant's payroll deductions for that Purchase Period
          to the purchase of whole shares of Common Stock at the purchase price
          in effect for the Purchase Period.

      C.  PURCHASE PRICE.  The purchase price per share at which Common Stock
          --------------
          will be purchased on the Participant's behalf on the last day of each
          Purchase Period shall be equal to eighty-five percent (85%) of the
          lower of (i) the Fair Market Value per share of Common Stock on the
          -----                                                              
          last business day immediately preceding the start date of that
          Purchase Period or (ii) the Fair Market Value per share of Common
          Stock on the Purchase Date.

      D.  NUMBER OF PURCHASABLE SHARES.  The number of shares of Common Stock
          ----------------------------                                       
          purchasable per Participant at the end of each Purchase Period shall
          be the number of whole shares obtained by dividing the amount
          collected from the Participant through payroll deductions during the
          Purchase Period by the purchase price in effect for that Purchase
          Period. However, the maximum number of shares of Common Stock
          purchasable per Participant on any one Purchase Date shall not exceed
          Three Hundred Seventy-Five (375) shares, subject to periodic
          adjustments in the event of certain changes in the Corporation's
          capitalization.

      E.  EXCESS PAYROLL DEDUCTIONS.  Any payroll deductions not applied to the
          -------------------------                                             
          purchase of shares of Common Stock at the end of any Purchase Period
          because they are not sufficient to purchase a whole share of Common
          Stock shall be held for the purchase of Common Stock on the next
          Purchase Date.  However, any payroll deductions not applied to the
          purchase of Common Stock by reason of the limitation on the maximum
          number of shares which the Participant may purchase on any one
          Purchase Date shall be promptly refunded.

      F.  TERMINATION OF PURCHASE RIGHT.  The following provisions shall govern
          -----------------------------                                        
          the termination of outstanding purchase rights:

          (I.)    A Participant may, at any time prior to the last day of the
                  Purchase Period, terminate his or her outstanding purchase
                  right by filing the appropriate form with the Plan
                  Administrator (or its designate), and no further payroll
                  deductions shall be collected from the Participant with
                  respect to the terminated purchase right. Any payroll
                  deductions collected on the Participant's behalf during the
                  Purchase


                                     A-5.
<PAGE>
 
                  Period in which such termination occurs shall be held for the
                  purchase of shares at the end of that Purchase Period.

          (II.)   The termination of such purchase right shall be irrevocable,
                  and the Participant may not subsequently rejoin the Purchase
                  Period for which the terminated purchase right was granted. In
                  order to resume participation in any subsequent Purchase
                  Period, such individual must re-enroll in the Plan (by making
                  a timely filing of the prescribed enrollment forms) prior to
                  the start date of that Purchase Period.

          (III.)  A Participant's purchase right shall immediately terminate
                  upon his or her cessation of Eligible Employee status for any
                  reason (other than death or Disability), and the payroll
                  deductions collected on behalf of the Participant pursuant to
                  the terminated purchase right shall be promptly refunded.

          (IV.)   Should the Participant cease to remain an Eligible Employee by
                  reason of death or Disability while his or her purchase right
                  remains outstanding, then such individual (or the personal
                  representative of the estate of a deceased Participant) shall
                  have the following election, exercisable up until the end of
                  the Purchase Period in which such cessation of Eligible
                  Employee status occurs:

                  . to withdraw all of the payroll deductions collected to date
                    on the Participant's behalf during that Purchase Period or

                  . to have such funds held for the purchase of shares at the
                    end of that Purchase Period.

                  In the absence of such a timely election, the Participant's
                  payroll deductions shall be refunded as soon as possible after
                  the close of the Purchase Period.

          (V.)    In no event may any payroll deductions be made on the
                  Participant's behalf following his/her cessation of Eligible
                  Employee status.

          (VI.)   Should the Participant cease to remain in active service by
                  reason of an approved unpaid leave of absence, then the
                  Participant shall have the election, exercisable up until the
                  last business day of the Purchase Period in which such leave
                  commences, to (a) withdraw all the payroll deductions
                  collected to date on the Participant's behalf during that
                  Purchase Period or (b) have such funds held for the purchase
                  of shares at the end of such Purchase Period. In no event,
                  however, shall any further payroll deductions be collected on
                  the


                                     A-6.
<PAGE>
 
                  Participant's behalf during such unpaid leave. Upon the
                  Participant's return to active service, his or her payroll
                  deductions under the Plan shall automatically resume at the
                  rate in effect at the time the leave began, provided the
                  Participant returns to service prior to the expiration date of
                  the Purchase Period in which such leave began.
                  
      G.  CHANGE IN CONTROL.  Each outstanding purchase right shall
          -----------------                                        
          automatically be exercised, immediately prior to the effective date of
          any Change in Control, by applying the payroll deductions of each
          Participant for the Purchase Period in which such Change in Control
          occurs to the purchase of whole shares of Common Stock at a purchase
          price per share equal to eighty-five percent (85%) of the lower of (i)
                                                                    -----
          the Fair Market Value per share of Common Stock on the last business
          day immediately preceding the start date of the Purchase Period in
          which such Change in Control occurs or (ii) the Fair Market Value per
          share of Common Stock immediately prior to the effective date of such
          Change in Control.

      H.  PRORATION OF PURCHASE RIGHTS.  Should the total number of shares of
          ----------------------------                                       
          Common Stock to be purchased pursuant to outstanding purchase rights
          on any particular date exceed the number of shares then available for
          issuance under the Plan, the Plan Administrator shall make a pro-rata
          allocation of the available shares on a uniform and nondiscriminatory
          basis, and the payroll deductions of each Participant, to the extent
          in excess of the aggregate purchase price payable for the Common Stock
          pro-rated to such individual, shall be refunded.

      I.  ASSIGNABILITY.  No purchase right granted under the Plan shall be
          -------------                                                    
          assignable or transferable by the Participant other than by will or by
          the laws of descent and distribution following the Participant's
          death, and during the Participant's lifetime the purchase right shall
          be exercisable only by the Participant.

      J.  STOCKHOLDER RIGHTS.  A Participant shall have no stockholder rights
          ------------------                                                 
          with respect to the shares subject to his or her outstanding purchase
          right until the shares are purchased on the Participant's behalf in
          accordance with the provisions of the Plan and the Participant has
          become a holder of record of the purchased shares.

      A Participant shall be issued, as soon as practicable after the end of
      each Purchase Period, a statement showing the number of shares purchased
      on the Participant's behalf for that Purchase Period and the purchase
      price paid per share. The Participant may at any time request the issuance
      of a stock certificate for any shares purchased on his or her behalf under
      the Plan. However, not more than one


                                     A-7.
<PAGE>
 
      certificate shall be issued per Participant for each Purchase Period of
      participation in the Plan, and such certificate may, upon the
      Participant's request, be issued in the names of the Participant and his
      or her spouse as community property or as joint tenants with right of
      survivorship. Alternatively, the stock certificate may be delivered to a
      designated stock brokerage account maintained for the Participant and held
      in "street name" in order to facilitate the subsequent sale of the
      purchased shares.

VIII. ACCRUAL LIMITATIONS

      A.  No Participant shall be entitled to accrue rights to acquire Common
          Stock pursuant to any purchase right outstanding under this Plan if
          and to the extent such accrual, when aggregated with (i) rights to
          purchase Common Stock accrued under any other purchase right granted
          under this Plan and (ii) similar rights accrued under other employee
          stock purchase plans (within the meaning of Code Section 423) of the
          Corporation or any Corporate Affiliate, would otherwise permit such
          Participant to purchase more than Twenty-Five Thousand Dollars
          ($25,000) worth of stock of the Corporation or any Corporate Affiliate
          (determined on the basis of the Fair Market Value of such stock on the
          date or dates such rights are granted) for each calendar year such
          rights are at any time outstanding.

      B.  For purposes of applying such accrual limitations to the purchase
          rights granted under this Plan, the following provisions shall be in
          effect:

          (I.)    The right to acquire Common Stock under each outstanding
                  purchase right shall accrue on the last day of the Purchase
                  Period for which such right is granted.

          (II.)   No right to acquire Common Stock under any outstanding
                  purchase right under the Plan shall accrue to the extent the
                  Participant has already accrued in the same calendar year the
                  right to acquire Common Stock under one (1) or more other
                  purchase rights at a rate equal to Twenty-Five Thousand
                  Dollars ($25,000) worth of Common Stock (determined on the
                  basis of the Fair Market Value of such stock on the date or
                  dates of grant) for each calendar year such rights were at any
                  time outstanding.

      C.  If by reason of such accrual limitations, any purchase right of a
          Participant does not accrue for a particular Purchase Period, then the
          payroll deductions collected on the Participant's behalf with respect
          to that purchase right shall be promptly refunded.


                                     A-8.
<PAGE>
 
      D.  In the event there is any conflict between the provisions of this
          Article and one or more provisions of the Plan or any instrument
          issued thereunder, the provisions of this Article shall be
          controlling.

IX.   EFFECTIVE DATE AND TERM OF THE PLAN

      A.  The Plan was adopted by the Board on October 19, 1995 and shall become
          effective on the Effective Date, provided the Plan is approved by the
                                           --------                            
          Corporation's stockholders at the 1995 Annual Meeting.  The Plan is
          intended to be the successor to the Predecessor Plan, and should such
          stockholder approval be obtained, the Predecessor Plan, together with
          all outstanding purchase rights thereunder, shall terminate with the
          November 30, 1995 quarterly purchase date thereunder, and no more
          purchase rights shall be granted or exercised and no shares of Common
          Stock shall be issued under the Predecessor Plan after the November
          30, 1995 purchase date.  In the event such stockholder approval is not
          obtained, then this Plan shall not be implemented, and the Predecessor
          Plan shall continue in full force and effect with its existing terms
          and provisions.

      B.  Unless sooner terminated by the Board, the Plan shall terminate upon
          the earliest of (i) the last business day in November 2005, (ii) the
              --------
          date on which all shares available for issuance under the Plan shall
          have been sold pursuant to purchase rights exercised under the Plan or
          (iii) the date on which all purchase rights are exercised in
          connection with a Change in Control. No further purchase rights shall
          be granted or exercised, and no further payroll deductions shall be
          collected, under the Plan following such termination.

X.    AMENDMENT OF THE PLAN

      The Board may alter, amend, suspend or discontinue the Plan at any time to
      become effective immediately following the close of any Purchase Period.
      However, the Board may not, without the approval of the Corporation's
      stockholders, (i) materially increase the number of shares of Common Stock
      issuable under the Plan or the maximum number of shares purchasable per
      Participant at the end of each Purchase Period, except for permissible
      adjustments in the event of certain changes in the Corporation's
      capitalization, (ii) alter the purchase price formula so as to reduce the
      purchase price payable for the shares of Common Stock purchasable under
      the Plan, or (iii) materially increase the benefits accruing to
      Participants under the Plan or materially modify the requirements for
      eligibility to participate in the Plan.

XI.   GENERAL PROVISIONS


                                     A-9.
<PAGE>
 
      A.  The Plan shall NOT be administered as a qualified employee stock
          purchase plan under Code Section 423. Accordingly, each Participant
          shall, in connection with his or her purchases of Common Stock under
          the Plan, satisfy all applicable federal, state and local income and
          employment tax withholding requirements.

      B.  All costs and expenses incurred in the administration of the Plan
          shall be paid by the Corporation.

      C.  Nothing in the Plan shall confer upon the Participant any right to
          continue in the employ of the Corporation or any Corporate Affiliate
          for any period of specific duration or interfere with or otherwise
          restrict in any way the rights of the Corporation (or any Corporate
          Affiliate employing such person) or of the Participant, which rights
          are hereby expressly reserved by each, to terminate such person's
          employment  at any time for any reason, with or without cause.

      D.  The provisions of the Plan shall be governed by the laws of the State
          of California without resort to that State's conflict-of-laws rules.


                                     A-10.
<PAGE>
 
                                   SCHEDULE A
                                   ----------

                         CORPORATIONS PARTICIPATING IN
                          EMPLOYEE STOCK PURCHASE PLAN
                            AS OF THE EFFECTIVE DATE
                            ------------------------


            Mycogen Corporation
            Mycogen Plant Sciences
            Mycogen Crop Protection
            Mycogen Canada, Inc.
            Mycogen Plant Sciences Puerto Rico Corporation



                                     A-11.
<PAGE>
 
                                   APPENDIX
                                   --------

The following definitions shall be in effect under the Plan:

  A.  BASE SALARY shall mean the (i) regular base salary paid to a Participant
      -----------                                                 
      by one or more Participating Companies during such individual's period of
      participation in the Plan plus (ii) any pre-tax contributions made by the
      Participant to any Code Section 401(k) salary deferral plan or any Code
      Section 125 cafeteria benefit program now or hereafter established by the
      Corporation or any Corporate Affiliate. Base Salary shall NOT include
      overtime payments, bonuses, commissions, profit-sharing distributions or
      other incentive-type payments, or any contributions (other than Code
      Section 401(k) or Code Section 125 contributions) made on the
      Participant's behalf by the Corporation or any Corporate Affiliate to any
      deferred compensation plan or welfare benefit program now or hereafter
      established.

  B.  BOARD shall mean the Corporation's Board of Directors.
      -----                                                 

  C.  CHANGE IN CONTROL shall mean a change in ownership or control of the
      -----------------                                                   
      Corporation effected through any of the following transactions:

      (I.)    a merger or consolidation in which securities possessing more than
              fifty percent (50%) of the total combined voting power of the
              Corporation's outstanding securities are transferred to a person
              or persons different from the persons holding those securities
              immediately prior to such transaction, or

      (II.)   the sale, transfer or other disposition of all or substantially
              all of the assets of the Corporation in complete liquidation or
              dissolution of the Corporation, or

      (III.)  the acquisition by any person or group of related persons (other
              than the Corporation or any person controlled by, controlling or
              under common control with the Corporation) of beneficial
              ownership of securities possessing more than fifty percent (50%)
              of the total combined voting power of the Corporation's
              outstanding securities pursuant to a tender or exchange offer
              made directly to the Corporation's stockholders.

  D.  CODE shall mean the Internal Revenue Code of 1986, as amended.
      ----                                                          

  E.  COMMON STOCK shall mean the Corporation's common stock.
      ------------                                           


                                     A-12.
<PAGE>
 
  F.  CORPORATE AFFILIATE shall mean any parent or subsidiary corporation of
      -------------------                                                   
      the Corporation (as determined in accordance with Code Section 424),
      whether now existing or subsequently established.

  G.  CORPORATION shall mean Mycogen Corporation, a California corporation,
      -----------                                                          
      and any corporate successor to all or substantially all of the assets
      or voting stock of Mycogen Corporation which shall by appropriate
      action adopt the Plan.

  H.  DISABILITY shall mean the Participant's inability, by reason of any
      ----------                                                         
      medically determinable physical or mental impairment expected to result in
      death or to be of continuous duration of twelve (12) months or more, to
      engage in any substantial gainful employment.

  I.  EFFECTIVE DATE shall mean December 1, 1995.  Any Corporate Affiliate
      --------------                                                      
      which becomes a Participating Corporation after such Effective Date shall
      designate a subsequent Effective Date with respect to its employee-
      Participants.

  J.  ELIGIBLE EMPLOYEE shall mean any person expected on a regularly-
      -----------------                                              
      scheduled basis expected to work than twenty (20) hours per week for more
      than five (5) months per calendar year in the employ of any Participating
      Corporation for earnings considered wages under Code Section 3401(a).

  K.  FAIR MARKET VALUE per share of Common Stock on any relevant date shall
      -----------------                                                     
      be determined in accordance with the following provisions:

      (I.)    If the Common Stock is at the time traded on the Nasdaq National
              Market, then the Fair Market Value shall be the closing selling
              price per share of Common Stock on the date in question, as such
              price is reported by the National Association of Securities
              Dealers on the Nasdaq National Market or any successor system.
              If there is no closing selling price for the Common Stock on the
              date in question, then the Fair Market Value shall be the closing
              selling price on the last preceding date for which such quotation
              exists.

      (II.)   If the Common Stock is at the time listed on any Stock Exchange,
              then the Fair Market Value shall be the closing selling price per


                                     A-13.
<PAGE>
 
              share on the date in question on the Stock Exchange determined by
              the Plan Administrator to be the primary market for the Common
              Stock, as such price is officially quoted in the composite tape
              of transactions on such exchange.  If there is no closing selling
              price for the Common Stock on the date in question, then the Fair
              Market Value shall be the closing selling price  on the last
              preceding date for which such quotation exists.

  L.  PARTICIPANT shall mean any Eligible Employee of a Participating
      -----------                                                    
      Corporation who is actively participating in the Plan.

  M.  PARTICIPATING CORPORATION shall mean the Corporation and such
      -------------------------                                    
      Corporate Affiliate or Affiliates as may be authorized from time to time
      by the Board to extend the benefits of the Plan to their Eligible
      Employees. The Participating Corporations in the Plan as of the Effective
      Date are listed in attached Schedule A.

  N.  PLAN shall mean the Corporation's 1995 Employee Stock Purchase Plan,
      ----                                                                
      as set forth in this document.

  O.  PLAN ADMINISTRATOR shall mean the Compensation Committee of the Board
      ------------------                                                   
      in its capacity as administrator of the Plan.

  P.  PREDECESSOR PLAN shall mean the Corporation's existing Employee Stock
      ----------------                                                     
      Purchase Plan.

  Q.  PURCHASE PERIOD shall mean each successive six (6) month period at the
      ---------------                                                       
      end of which there shall be purchased shares of Common Stock on behalf of
      each Participant.

  R.  PURCHASE DATE shall mean the last business day of each Purchase Period
      -------------                                                         
      and shall accordingly occur on the last business day of May and November
      each year.

  S.  SECURITIES ACT shall mean the Securities Act of 1933, as amended.
      --------------                                                   

  T.  STOCK EXCHANGE shall mean either the American Stock Exchange or the
      --------------                                                     
      New York Stock Exchange.



                                     A-14.