Exhibit 24

                               POWER OF ATTORNEY

        The undersigned, as a Section 16 reporting person of Pfenex Inc. (the
"Company"), hereby constitutes and appoints Evert Schimmelpennink, Marilyn Bock,
Daniel Koeppen, Zachary Myers and Cassie Wetzel the undersigned's true and
lawful attorneys-in-fact to:

        1.   complete and execute Forms 3, 4 and 5 and other forms and all
             amendments thereto as such attorneys-in-fact shall in their
             discretion determine to be required or advisable pursuant to
             Section 16 of the Securities Exchange Act of 1934 (as amended) and
             the rules and regulations promulgated thereunder, or any successor
             laws and regulations, as a consequence of the undersigned's
             ownership, acquisition or disposition of securities of the Company;
             and

        2.   do all acts necessary in order to file such forms with the
             Securities and Exchange Commission, any securities exchange or
             national association, the Company and such other person or agency
             as the attorneys-in-fact shall deem appropriate.

        The undersigned, as a Section 16 reporting person of the Company, hereby
revokes any and all prior powers of attorney previously signed by the
undersigned.

        The undersigned hereby ratifies and confirms all that said attorneys-in-
fact and agent shall do or cause to be done by virtue hereof.  The undersigned
acknowledges that the foregoing attorneys-in-fact, in serving in such capacity
at the request of the undersigned, is not assuming, nor is the Company assuming,
any of the undersigned's responsibilities to comply with Section 16 of the
Securities Exchange Act of 1934 (as amended).

        This Power of Attorney shall remain in full force and effect until the
undersigned is no longer required to file Forms 3, 4 and 5 with respect to the
undersigned's holdings of and transactions in securities issued by the Company,
unless earlier revoked by the undersigned in a signed writing delivered to the
Company and the foregoing attorneys-in-fact.

        IN WITNESS WHEREOF, the undersigned has caused this Power of Attorney to
be executed as of this 6th day of January, 2020.


                                       Signature:  /s/ Shawn Scranton
                                                 -------------------------------

                                       Print Name:  Shawn Scranton
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