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                                                                   EXHIBIT 10.18

                            UNSECURED PROMISSORY NOTE

U.S. $1,669,540.07                                         San Diego, California
                                                                    June 2, 2004

      FOR VALUE RECEIVED, the undersigned, Stratagene Corporation, a Delaware
corporation ("OBLIGOR"), hereby promises to pay to Joseph A. Sorge, M.D., an
individual ("HOLDER"), the principal sum of One Million Six Hundred Sixty Nine
Thousand Five Hundred Forty and 07/100 Dollars ($1,669,540.07), together with
interest thereon at the times and in the amounts set forth below.

      1. Interest Rate. The unpaid principal amount hereof shall bear interest
from the date hereof until all principal and accrued and unpaid interest hereon
is paid in full at a rate of 3.89% per annum, compounded annually (which rate is
the lowest applicable rate permissible under the Internal Revenue Code of 1986,
as amended, without the imputation of interest income or expense). Interest
shall be payable in arrears on the payment dates set forth in Section 2 hereof
based on a year consisting of twelve months, each consisting of 30 days.

      2. Payment.

            (a) All payments of principal and interest under this Note shall be
in lawful money of the United States of America. All payments will be applied
first to the cost of collection, if any, then to accrued and unpaid interest,
and thereafter to principal. In the event that a payment hereunder becomes due
and payable on a day other than a business day on which banks in the State of
California are open for business, such payment shall become due and payable on
the next succeeding business day on which banks in the State of California are
open for business. All payments will be by wire transfer of immediately
available funds to an account designated in writing by Holder.

            (b) Obligor will pay 1/39th of the outstanding principal balance of
this Note and the accrued but unpaid interest thereon (the "TOTAL AMOUNT") on
July 1, 2004 (such payment, the "MONTHLY TOTAL AMOUNT"). Thereafter, Obligor
shall pay principal installments equal to the Monthly Total Amount and interest
installments equal to the amount of the unpaid interest accrued on the Total
Amount on the first day of the subsequent month, and continuing on the same day
of each month thereafter until the Maturity Date (as defined below), at which
time the entire remaining balance of principal and accrued interest on the Note
shall be due and payable. As used herein, the term "MATURITY DATE" shall mean
September 1, 2007.

      3. Default. Each of the following events shall be an "EVENT OF DEFAULT"
hereunder:

            (a) Obligor fails to make within five (5) business days of when due
any payment of principal, interest or other amounts owing hereunder;

            (b) Obligor files any petition or action for relief under any
bankruptcy, reorganization, insolvency or moratorium law or any other law for
the relief of, or relating to, debtors, now or hereafter in effect, or makes any
assignment for the benefit of creditors or takes any corporate action in
furtherance of any of the foregoing; or

            (c) an involuntary petition is filed against Obligor (unless such
petition is dismissed or discharged within sixty (60) days of filing) under any
bankruptcy statute now or hereafter in effect, or a custodian, receiver,
trustee, assignee for the benefit of creditors (or other similar official) is
appointed to take possession, custody or control of any property of Obligor.
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Upon the occurrence of an Event of Default hereunder, all unpaid principal,
accrued interest and other amounts owing hereunder shall, at the option of
Holder, and, in the case of an Event of Default pursuant to clauses (b) or (c)
above, automatically, be immediately due, payable and collectible by Holder
pursuant to applicable law.

      4. Prepayment. This Note may be prepaid, in whole or in part, at any time
and from time to time, without penalty or premium; provided, however, that any
such prepayment shall first be applied against any accrued and unpaid interest,
and the balance shall be applied against the principal.

      5. Usury. Anything in this Note to the contrary notwithstanding, if at any
time the rate of interest on the Note together with all fees and charges, if any
(collectively, the "CHARGES"), contracted for, charged, received, taken or
reserved by Holder which may be treated as interest under applicable law,
computed over the full term of the Note, exceeds the maximum legal limit (if any
such limit is applicable) under United States federal law or state law (to the
extent not preempted by federal law, if any), now or hereafter governing the
interest payable on the Note (the "MAXIMUM RATE"), then the rate of interest on
the Note, together with all Charges, shall be limited to the Maximum Rate. If
from any circumstances Holder shall ever receive as interest an amount which
would exceed the Maximum Rate, such amount which would be excessive interest
shall be applied to the reduction of the unpaid principal balance hereunder
(whether or not due and payable) and not to the payment of interest.

      6. Costs and Expenses of Collection. If this Note is collected by or
through an attorney at law as a result of a failure of Obligor to pay, when due
hereunder, any payment of principal of or other amount due under this Note,
Obligor shall pay all of the Holder's reasonably incurred costs of collection,
including, without limitation, Holder's attorneys' fees.

      7. Miscellaneous.

            (a) Presentment, demand, protest, notices of protest, dishonor and
non-payment of this Note and all notices of every kind are hereby waived. To the
extent permitted by applicable law, the defense of the statute of limitations is
hereby waived by Obligor.

            (b) No single or partial exercise of any power hereunder shall
preclude other or further exercise thereof or the exercise of any other power.
No delay or omission on the part of the holder hereof in exercising any right
hereunder shall operate as a waiver of such right or of any other right under
this Note. No waiver of any breach of any of the covenants or conditions of this
Note shall be construed to be a waiver of or acquiescence in or a consent to any
previous or subsequent breach of the same or any other condition or covenant.

            (c) No right, power or remedy conferred upon or reserved to the
holder hereof by this Note is intended to be exclusive of any other right, power
or remedy, but each and every such right, power and remedy shall be cumulative
and concurrent and shall be in addition to any other right, power and remedy
given hereunder or now or hereafter existing at law or in equity or by statute.
Every power or remedy given by this Note to the holder hereof or to which such
holder may be entitled may be exercised from time to time and as often as may be
deemed expedient by such holder.

            (d) This Note shall be governed by and construed in accordance with
the internal laws of the State of California without regard to principles of
conflicts of laws.

            (e) Time is hereby declared to be of the essence of this Note and of
every part hereof. When the context and construction so require, all words used
in the singular herein shall be deemed to have been used in the plural and the
masculine shall include the feminine and the neuter and


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vice versa. Titles and sections are for convenience only and neither limit nor
amplify the provisions of this Note, and all references herein to sections or
paragraphs shall refer to the corresponding sections or paragraphs of this Note
unless specific reference is made to such sections or paragraphs of another
document or instrument.

            (f) If any provision of this Note or the application thereof to any
person or circumstance shall be invalid or unenforceable to any extent, the
remainder of this Note and the application of such provision to other persons or
circumstances shall not be affected thereby and shall be enforced to the maximum
extent permitted by law.

            (g) Any and all notices required by this Note shall be personally
delivered or sent by certified mail, return receipt requested, or sent by
machine confirmed facsimile transmission, addressed to a party at its address
set forth below, or at such other address as it may designate to the other party
in accordance with this paragraph. A notice shall be deemed effective when
delivered if personally delivered, three (3) business days following mailing if
sent by certified mail, return receipt requested, and upon machine confirmation
of transmission when sent by facsimile.

               If to Obligor:                         If to Holder:

               Stratagene Corporation                 Joseph A. Sorge, M.D.
               11011 North Torrey Pines Road          144 10th Street
               La Jolla, California  92037            Del Mar, California  92014
               Attention:  Chief Financial Officer    Facsimile:  (858) 509-4899
               Facsimile:  (858) 535-0071

            The parties hereto may change their addresses by giving notice
thereof to the other parties hereto in conformity with this section.

            (h) This Note may not be assigned by Obligor without the prior
written consent of the holder hereof. This Note or any interest hereunder may be
freely assigned by the holder hereof without the consent of Obligor. Subject to
the foregoing, this Note shall inure to the benefit of the holder hereof, its
successors, assigns and representatives and shall bind Obligor, its successors,
assigns and representatives. This Note may not be modified, amended or
terminated except by a written agreement signed by Obligor and the holder
hereof.

            (i) No director, officer or employee, as such, of Obligor shall have
any liability for any obligations of Obligor under the Note or for any claim
based on, in respect of or by reason of such obligation or its creation. By
accepting this Note, Holder waives and releases all such liability. The waiver
and release are part of the consideration for the Note.

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      IN WITNESS WHEREOF, Obligor has executed and delivered this Note effective
as of the date first written above.

                                          OBLIGOR

                                          STRATAGENE CORPORATION


                                          By: /s/ RONNI L. SHERMAN
                                              ----------------------------------
                                                  Ronni L. Sherman
                                                  Executive Vice President and
                                                  General Counsel


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