EXHIBIT 10.17

THE BANK OF CALIFORNIA


                      FOURTH AMENDMENT TO CREDIT AGREEMENT

THIS FOURTH AMENDMENT ("Amendment") is made effective as of the 31st day of May,
1995, by and between EMCON, a California  corporation  ("Borrower") and THE BANK
OF CALIFORNIA, N.A., a national banking association ("Bank").

RECITALS

A.       Borrower  is  currently  indebted  to Bank  pursuant  to the  terms and
         conditions of that certain Credit Agreement dated September 20, 1991 as
         amended May 31, 1993 and June 2, 1994 (the "Agreement");

B.       Borrower and Bank have agreed to amend the Agreement to reflect certain
         changes in the terms and conditions set forth therein.

NOW, THEREFORE, the parties hereto agree as follows:

1.       The definition of  "Termination  Date"  appearing in Article One of the
         Agreement  is  hereby  deleted  in  its  entirety,  and  the  following
         substituted therefor:


               "Termination  Date'  means the  earlier of (a) the date
               Bank may terminate  making Advances or extending credit
               pursuant to the rights of Bank under  Article 7; or (b)
               May 31, 1996 for the Line of Credit."

2.       Section 3.8 of the Agreement is hereby deleted in its entirety, and the
         following substituted therefor:


               "3.8 No Subsidiaries.  Borrower is not a majority owner
               of or in a control relationship with any other business
               entity,  except  Columbia  Analytical  Services,  Inc.;
               EMCON Alaska,  Inc.; Monterey Landfill Gas Corporation;
               Yolo  Landfill  Gas  Corporation;  Aquila  Construction
               Company;    Wehran    Construction,     Inc.;    Wehran
               Technological  Services,  Inc.;  Wehran-New York, Inc.;
               WECON  Services  Corporation;  Wehran Puerto Rico,  the
               share of each of which are owned 100% by Borrower,  and
               ET  Environmental  Corp., the shares of which are owned
               50% by Borrower (collectively, the `Subsidiaries').




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                              CONDITIONS PRECEDENT

Required Delivery.  The obligation of Bank to extend any Amendment is subject to
the  condition  that,  on or before the date of any Advance and or  extension of
credit,  there shall have been  delivered  to Bank,  each in form and  substance
satisfactory to Bank, and duly executed as required by Bank:

     (a)      The Amendment;

     (b)      Any and all Loan  Documents  Bank may require to evidence any Lien
              granted to Bank in connection with this Amendment;

     (c)      Payment in full no later than June 15, 1995, a non-refundable  fee
              of $15,000.00 for this Agreement;

     (d)      Such other  documents,  instruments or agreements Bank may require
              to evidence the Amendment.

                          GENERAL AMENDMENT PROVISIONS

         A. Except as specifically  provided herein, all terms and conditions of
the Agreement  remain in full force and effect,  without waiver or modification.
All terms defined in the Agreement shall have the same meaning when used in this
Amendment,  and this  Amendment and the Agreement  shall be read together as one
document. Where any provisions of the Agreement amended by this Amendment appear
in a  promissory  note  tied  to the  Agreement,  the  same  provisions  in said
promissory note shall be deemed likewise amended.

         B.  Borrower  hereby  confirms  all   representations   and  warranties
contained in the  Agreement  and  reaffirms  all  covenants  set forth  therein.
Further, Borrower certifies that, as of the date of this Amendment, there exists
no Event of Default as defined in the Agreement, nor any condition, act or event
which with the giving of notice or the passage of time or both would  constitute
an Event of Default.

IN WITNESS  WHEREOF,  the parties  hereto have caused this  Amendment  to become
effective as of the date and year first written above.

THE BANK OF CALIFORNIA, N.A.


By:  /s/ Marie Wiseman
     ---------------------------
     Marie Wiseman
     Vice President

EMCON,
a California corporation


By:  /s/ Eugene M. Herson
     ---------------------------
     Eugene M. Herson
     President


By:  /s/ R. Michael Momboisse
     ---------------------------
     R. Michael Momboisse
     Chief Financial Officer




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THE BANK OF CALIFORNIA


                                  EXHIBIT "A"
                        TO ADDENDUM TO CREDIT AGREEMENT
                             BORROWER SUBSIDIARIES

Columbia Analytical  Services,  Inc.; EMCON Alaska,  Inc.; Monterey Landfill Gas
Corporation; Yolo Landfill Gas Corporation;  Aguila Construction Company; Wehran
Construction,  Inc.; Wehran Technological Services, Inc.; Wehran-New York, Inc.;
WECON Services Corporation; Wehran Puerto Rico; and ET Environmental Corp.

Initials:         /s/
               --------






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