MONSANTO CO
S-4/A, 2000-01-25
CHEMICALS & ALLIED PRODUCTS
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<PAGE>


 As Filed with the Securities and Exchange Commission on January 25, 2000
                                                     Registration No. 333-73233
- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------

                      SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C. 20549

                                --------------

                             AMENDMENT NO. 4
                                      TO

                                   FORM S-4
                            REGISTRATION STATEMENT
                                     Under
                          The Securities Act of 1933

                                --------------

                               MONSANTO COMPANY
            (Exact name of Registrant as specified in its charter)

               Delaware                              43-0420020
    (State or other jurisdiction of     (I.R.S. Employer Identification No.)
    incorporation or organization)

                                --------------

                         800 North Lindbergh Boulevard
                           St. Louis, Missouri 63167
                                (314) 694-1000
  (Address, including zip code, and telephone number, including area code, of
                   Registrant's principal executive office)

                                --------------

                           R. William Ide III, Esq.
                               Monsanto Company
                         800 North Lindbergh Boulevard
                           St. Louis, Missouri 63167
                                (314) 694-1000
(Name, address, including zip code, and telephone number, including area code,
                             of agent for service)

                                --------------

                                  Copies to:

       Judith A. Reinsdorf                         Robert F. Wall
          Sonya Meyers Davis                      Terrence R. Brady
           Monsanto Company                       Winston & Strawn
     800 North Lindbergh Boulevard              35 West Wacker Drive
       St. Louis, Missouri 63167               Chicago, Illinois 60601
            (314) 694-1000                         (312) 558-5600

                                --------------

  Approximate Date of Commencement of Proposed Sale to Public: As soon as
practicable after this Registration Statement becomes effective.

  If the securities being registered on this Form are being offered in
connection with the formation of a holding company and there is compliance
with General Instruction G, check the following box: [_]

  If this form is filed to register additional securities for an offering
pursuant to Rule 462(b) under the Securities Act, check the following box and
list the Securities Act registration statement number of the earlier effective
registration statement for the same offering: [_]

  If this form is a post-effective amendment filed pursuant to Rule 462(d)
under the Securities Act, check the following box and list the Securities Act
registration statement number of the earlier effective registration statement
for the same offering: [_]

                                --------------

  The Registrant hereby amends this Registration Statement on such date or
dates as may be necessary to delay its effective date until the Registrant
shall file a further amendment which specifically states that this
Registration Statement shall thereafter become effective in accordance with
Section 8(a) of the Securities Act of 1933 or until this Registration
Statement shall become effective on such date as the Commission, acting
pursuant to said Section 8(a), may determine.

- -------------------------------------------------------------------------------
- -------------------------------------------------------------------------------
<PAGE>


               SUBJECT TO COMPLETION, DATED JANUARY 25, 2000

PROSPECTUS

[Monsanto logo]

                                Monsanto Company

                               Exchange Offer for

                       $500,000,000 5.375% Notes due 2001
                       $600,000,000 5.750% Notes due 2005
                       $200,000,000 5.875% Notes due 2008
                    $500,000,000 6.500% Debentures due 2018
                    $700,000,000 6.600% Debentures due 2028

                          Terms of the Exchange Offer

  .  The exchange offer expires at 5:00 p.m., New York City time, on      ,
     2000, unless Monsanto, in its sole discretion, extends the exchange
     offer to allow additional tenders of outstanding notes and debentures.

  .  All outstanding notes and debentures that are validly tendered and not
     validly withdrawn will be exchanged.

  .  Tenders of outstanding notes and debentures may be withdrawn at any time
     prior to the expiration of the exchange offer.

  .  The exchange offer is not subject to any condition, other than that the
     exchange offer not violate applicable law or any applicable
     interpretation of the staff of the Securities and Exchange Commission.

  .  Monsanto will not receive any proceeds from the exchange offer.

  .  The exchange of notes and debentures will not be a taxable exchange for
     U.S. federal income tax purposes.

  .  The terms of the registered notes and debentures and the outstanding
     notes and debentures are substantially identical, except that the
     outstanding notes and debentures are subject to transfer restrictions
     and have registration rights.

  .  There is no existing market for the registered notes and debentures, and
     Monsanto does not intend to apply for their listing on any securities
     exchange.

   For a discussion of some factors that you should consider before tendering
your outstanding notes and debentures in the exchange offer, see "Risk Factors"
beginning on page 7.

   Neither the Securities and Exchange Commission nor any state securities
commission has approved or disapproved of these securities or passed upon the
adequacy or accuracy of this prospectus. Any representation to the contrary is
a criminal offense.

   The information in this prospectus is not complete and may be changed.
Monsanto may not sell these securities until the registration statement filed
with the Securities and Exchange Commission is effective. This prospectus is
not an offer to sell these securities and it is not soliciting an offer to buy
these securities in any state where the offer or sale is not permitted.

                               ----------------

              The date of this Prospectus is         , 2000.
<PAGE>

   Each broker-dealer that receives registered debt for its own account
pursuant to the exchange offer must acknowledge that it will deliver a
prospectus in connection with any resale of such registered debt. Each letter
of transmittal states that by so acknowledging and by delivering a prospectus,
a broker-dealer will not be deemed to admit that it is an "underwriter" within
the meaning of the Securities Act. This prospectus, as it may be amended or
supplemented from time to time, may be used by a broker-dealer in connection
with resales of registered debt received in exchange for outstanding debt
acquired by such broker-dealer as a result of market-making activities or other
trading activities. Monsanto has agreed that, ending on the close of business
on the 180th day following the expiration date of the exchange offer, it will
make this prospectus available to any broker-dealer for use in connection with
any such resale. See "Plan of Distribution."

   You should rely on the information contained, or incorporated by reference,
in this prospectus. Monsanto has not authorized anyone to provide you with
different information. Monsanto is not making an offer of these securities in
any state where the offer is not permitted. You should not assume that the
information contained in this prospectus is accurate as of any date other than
the date on the front cover of this prospectus.

                               ----------------

                               TABLE OF CONTENTS

<TABLE>
<CAPTION>
                                                                            Page
                                                                            ----
<S>                                                                         <C>
Prospectus Summary.........................................................   1
Risk Factors...............................................................   7
Use of Proceeds............................................................  11
The Exchange Offer.........................................................  12
Description of the Registered Debt.........................................  20
United States Federal Tax Consequences.....................................  30
Plan of Distribution.......................................................  34
Where You Can Find More Information........................................  35
Incorporation of Certain Documents by Reference............................  35
Validity of the Registered Debt............................................  37
Experts....................................................................  37
</TABLE>

                               ----------------

   Trademarks and service marks owned or licensed by Monsanto and its
subsidiaries are indicated by special type throughout this prospectus.
<PAGE>

                               PROSPECTUS SUMMARY

   The following summary is qualified in its entirety by reference to the more
detailed information and consolidated financial information appearing elsewhere
in or incorporated by reference into this prospectus.

                                  The Company

   Monsanto is a life sciences company, committed to finding solutions to the
growing global needs for food and health by applying common forms of science
and technology among agriculture, nutrition and health. Monsanto makes,
researches and markets high-value agricultural products, pharmaceuticals and
nutrition-based health products.

   Monsanto was incorporated in 1933 under Delaware law and is the successor to
a Missouri corporation, Monsanto Chemical Works, organized in 1901. Monsanto's
principal executive offices are located at 800 North Lindbergh Boulevard, St.
Louis, Missouri 63167 (telephone number (314) 694-1000) and its internet
address is http://www.monsanto.com.

                         Purpose of the Exchange Offer

   On December 9, 1998, Monsanto sold, through a private placement exempt from
the registration requirements of the Securities Act of 1933, $500 million of
its 5.375% Notes due 2001, $600 million of its 5.750% Notes due 2005, $200
million of its 5.875% Notes due 2008, $500 million of its 6.500% Debentures due
2018 and $700 million of its 6.600% Debentures due 2028. We refer to these five
series of outstanding notes and debentures as "outstanding debt" in this
prospectus. Monsanto used the net proceeds from the sale of the outstanding
debt to finance or refinance seed company acquisitions, including to refinance
its outstanding commercial paper as it became due, and for working capital
purposes. Pending use for the foregoing purposes, Monsanto invested the
proceeds in short-term investment grade marketable securities or money market
obligations.

   Simultaneously with the private placement, Monsanto entered into a
registration rights agreement with the initial purchasers of the outstanding
debt. Under the registration rights agreement, Monsanto was required to use its
reasonable efforts to cause a registration statement for substantially
identical notes and debentures, to be issued in exchange for the outstanding
debt, to become effective on or before June 7, 1999. We refer to the notes and
debentures to be registered under this exchange offer registration statement as
"registered debt" in this prospectus. The registration rights agreement
requires Monsanto to pay liquidated damages to the holders of outstanding debt
from June 8, 1999 until it consummates the exchange offer. You may exchange
your outstanding debt for registered debt in this exchange offer. You should
read the discussion under the heading "Summary of Terms of the Registered Debt"
and "Description of the Registered Debt" for further information regarding the
registered debt.

   Monsanto did not register the outstanding debt under the Securities Act or
any state securities laws, nor does it intend to after the exchange offer. As a
result, the outstanding debt may only be transferred in limited circumstances
under the securities laws. If the holders of the outstanding debt do not
exchange their notes and debentures in the exchange offer, they lose their
right to have the outstanding debt registered under the Securities Act, subject
to certain limitations. Anyone who still holds outstanding debt after the
exchange offer may be unable to resell such notes and debentures.

   However, Monsanto believes that holders of the outstanding debt may resell
the registered debt without complying with the registration and prospectus
delivery provisions of the Securities Act, if they meet certain

                                       1
<PAGE>

conditions. You should read the discussion under the headings "Summary of the
Exchange Offer" and "The Exchange Offer" for further information regarding the
exchange offer and resales of the registered debt.

                         Summary of the Exchange Offer

Registration Rights.......  Monsanto sold the outstanding debt on December 9,
                            1998 to Salomon Smith Barney Inc. and Goldman,
                            Sachs & Co., the initial purchasers. The initial
                            purchasers then sold the outstanding debt to
                            institutional investors. Simultaneously with the
                            initial sale of the outstanding debt, Monsanto
                            entered into a registration rights agreement with
                            the initial purchasers, which provides for the
                            exchange offer.

                            You may exchange your outstanding debt for
                            registered debt, which has substantially identical
                            terms. The exchange offer satisfies your rights
                            under the registration rights agreement. After the
                            exchange offer is over, you will not be entitled to
                            any exchange or registration rights with respect to
                            your outstanding debt. Therefore, if you do not
                            exchange your outstanding debt, you will not be
                            able to reoffer, resell or otherwise dispose of
                            your outstanding debt unless (1) you comply with
                            the registration and prospectus delivery
                            requirements of the Securities Act, or (2) you are
                            exempt from such Securities Act requirements.

The Exchange Offer........  Monsanto is offering to exchange $500 million total
                            principal amount of its 5.375% Notes due 2001, $600
                            million total principal amount of its 5.750% Notes
                            due 2005, $200 million total principal amount of
                            its 5.875% Notes due 2008, $500 million total
                            principal amount of its 6.500% Debentures due 2018
                            and $700 million total principal amount of its
                            6.600% Debentures due 2028, which have been
                            registered under the Securities Act, for,
                            respectively, your outstanding 5.375% Notes due
                            2001, 5.750% Notes due 2005, 5.875% Notes due 2008,
                            6.500% Debentures due 2018 or 6.600% Debentures due
                            2028 sold in the December 1998 private offering. To
                            exchange your outstanding debt, you must properly
                            tender it, and Monsanto must accept it. Monsanto
                            will exchange all outstanding debt that you validly
                            tender and do not validly withdraw. Monsanto will
                            issue registered debt at or promptly after the end
                            of the exchange offer.

Resales...................  Monsanto believes that you can offer for resale,
                            resell or otherwise transfer the registered debt
                            without complying with the registration and
                            prospectus delivery requirements of the Securities
                            Act if:

                               .  you acquire the registered debt in the
                                  ordinary course of your business;

                               .  you are not participating, do not intend to
                                  participate, and have no arrangement or
                                  understanding with any person to
                                  participate, in the distribution of the
                                  registered debt; and

                               .  you are not an "affiliate" of Monsanto, as
                                  defined in Rule 405 of the Securities Act.

                                       2
<PAGE>


                            If any of these conditions is not satisfied and you
                            transfer any registered debt without delivering a
                            proper prospectus or without qualifying for a
                            registration exemption, you may incur liability
                            under the Securities Act. Monsanto will not assume
                            or indemnify you against such liability.

                            Each broker-dealer that receives registered debt
                            for its own account in exchange for outstanding
                            debt, where such outstanding debt was acquired by
                            such broker-dealer as a result of market-making
                            activities or other trading activities, must
                            acknowledge that it will deliver a prospectus in
                            connection with any resale of such registered debt.
                            See "Plan of Distribution." A broker-dealer may use
                            this prospectus for an offer to resell, a resale or
                            other retransfer of the registered debt.

Expiration Date...........  The exchange offer expires at 5:00 p.m., New York
                            City time, on           , 2000, unless Monsanto, in
                            its sole discretion, extends the exchange offer to
                            allow additional tenders of outstanding debt.

Liquidated Damages........  Under the registration rights agreement, Monsanto
                            is required to pay liquidated damages to the
                            holders of outstanding debt from June 8, 1999,
                            until it consummates the exchange offer.

Conditions to the           The exchange offer is subject to conditions, some
 Exchange Offer...........  of which Monsanto may waive. Such conditions are
                            more fully described later in this prospectus under
                            "The Exchange Offer--Conditions to the Exchange
                            Offer."

Procedures for Tendering
 Outstanding Debt.........  Monsanto issued the outstanding debt as global
                            securities. When the outstanding debt was issued,
                            Monsanto deposited it with The Bank of New York, as
                            depositary. The Bank of New York established a
                            certificateless depositary interest in each note
                            and debenture, which represents a 100% interest in
                            the notes and debentures, in the name of Cede &
                            Co., the nominee of The Depository Trust Company,
                            commonly known as "DTC". Beneficial interests in
                            the outstanding debt, which are held by direct or
                            indirect participants in DTC through the
                            certificateless depositary interest, are shown on
                            records maintained in book-entry form by DTC.

                            You may tender your outstanding debt through book-
                            entry transfer in accordance with DTC's Automated
                            Tender Offer Program, or "ATOP". To tender your
                            outstanding debt by a means other than ATOP, a
                            letter of transmittal must be completed and signed
                            according to the instructions contained in such
                            letter. The letter of transmittal and any other
                            documents required by the letter of transmittal
                            must be delivered to the exchange agent by mail,
                            facsimile, hand delivery or overnight carrier. In
                            addition, you must deliver your outstanding debt to
                            the exchange agent or comply with the procedures
                            for guaranteed delivery. See "The Exchange Offer--
                            Procedures for Tendering Outstanding Debt" for more
                            information.

                                       3
<PAGE>


                            Do not send letters of transmittal and certificates
                            representing outstanding debt, if any, to Monsanto.
                            Send these documents only to the exchange agent.
                            See "The Exchange Offer--Exchange Agent" for more
                            information.

Special Procedures for
 Beneficial Owners........  If you are a beneficial owner whose outstanding
                            debt is registered in the name of a broker, dealer,
                            commercial bank, trust company or other nominee,
                            and you wish to tender your outstanding debt in the
                            exchange offer, please contact the registered
                            holder as soon as possible and instruct them to
                            tender on your behalf and comply with the
                            instructions set forth elsewhere in this
                            prospectus.

Withdrawal Rights.........  You may withdraw the tender of your outstanding
                            debt at any time before 5:00 p.m., New York City
                            time, on           , 2000, unless Monsanto extends
                            the date.

Appraisal or Dissenters'
 Rights...................  Holders of outstanding debt do not have any
                            appraisal or dissenters' rights in the exchange
                            offer. If you do not tender your outstanding debt
                            or Monsanto does not accept your tender because,
                            among other things, you invalidly tendered it, you
                            will not be entitled to any further registration
                            rights under the registration rights agreement,
                            except under limited circumstances. However, your
                            notes and debentures will remain outstanding and
                            entitled to the benefits of the indenture.

Federal Income Tax
 Considerations...........  The exchange of outstanding notes and debentures is
                            not a taxable exchange for United States federal
                            income tax purposes. You will not recognize any
                            taxable gain or loss or any interest income as a
                            result of the exchange. For additional information
                            regarding federal income tax considerations, you
                            should read the discussion under the heading
                            "United States Federal Tax Consequences."

Use of Proceeds...........  Monsanto will not receive any proceeds from the
                            issuance of the registered debt, and Monsanto will
                            pay the expenses of the exchange offer.

Exchange Agent............  The Bank of New York is serving as exchange agent
                            in the exchange offer. The mailing address of the
                            exchange agent is The Bank of New York, 101 Barclay
                            Street, 7E, New York, New York 10286, Attention:
                            Reorganization Department. Deliveries by hand or
                            overnight courier should be addressed to The Bank
                            of New York, 101 Barclay Street, 7E, Corporate
                            Trust Services Window, Ground Level, New York, New
                            York 10286, Attention: Reorganization Department.
                            For information about the exchange offer, call the
                            exchange agent at telephone number: (212) 815-6333
                            or facsimile number: (212) 815-4699.

                                       4
<PAGE>


                    Summary of Terms of the Registered Debt

   The form and terms of the registered debt are the same as the form and terms
of the outstanding debt, except that the registered debt will be registered
under the Securities Act. As a result, the registered debt will not bear
legends restricting its transfer and will not contain the registration rights
and liquidated damages provisions contained in the outstanding debt. The
registered debt represents the same debt as the outstanding debt. Both the
outstanding debt and the registered debt are governed by the same indenture.

Aggregate Amount..........  $500,000,000 principal amount of 5.375% Notes due
                            2001, $600,000,000 principal amount of 5.750% Notes
                            due 2005, $200,000,000 principal amount of 5.875%
                            Notes due 2008, $500,000,000 principal amount of
                            6.500% Debentures due 2018, and $700,000,000
                            principal amount of 6.600% Debentures due 2028.

Interest..................  Interest will accrue on the registered debt from
                            the date of initial issuance and will be payable on
                            June 1 and December 1 of each year, beginning June
                            1, 2000. Holders of registered debt will receive
                            interest on June 1, 2000 from the date of initial
                            issuance of the registered debt, plus an amount
                            equal to the accrued, but unpaid, interest on the
                            outstanding debt.

Ranking...................  The registered debt will be senior unsecured
                            obligations of Monsanto and will rank equally with
                            all other senior unsecured and unsubordinated
                            indebtedness of Monsanto.

Optional Redemption.......  The Notes due 2001 are not redeemable prior to
                            maturity. The other series of registered debt will
                            be redeemable as a whole or in part, at the option
                            of Monsanto, at any time at a redemption price
                            equal to the greater of (1) 100% of the principal
                            amount of the registered debt to be redeemed or (2)
                            the sum of the present values of the remaining
                            scheduled principal and interest payments
                            discounted, on a semiannual basis, at a rate equal
                            to the sum of the applicable Treasury Rate (as
                            defined herein) and 15 basis points for the Notes
                            due 2005, 20 basis points for the Notes due 2008
                            and the Debentures due 2018, and 25 basis points
                            for the Debentures due 2028, plus, in each case,
                            accrued interest to the date of redemption.

Sinking Fund..............  None.

Use of Proceeds...........  Monsanto will not receive any cash proceeds in the
                            exchange offer.

Form of the Registered
 Debt.....................  The registered debt will be represented by one or
                            more permanent global securities in bearer form
                            deposited with The Bank of New York, as book-entry
                            depositary, for the benefit of DTC. You will not
                            receive notes or debentures in registered form
                            unless one of the events set forth under the
                            heading "Description of the Registered Debt--Global
                            Securities" and "--Certificated Securities" occurs.
                            Instead, beneficial interests in the registered
                            debt will be shown on, and transfers of these
                            interests will be effected only through, records
                            maintained in book-entry form by DTC with respect
                            to its participants.

Absence of a Public
 Market for the
 Registered Debt..........  Monsanto does not intend to apply for a listing of
                            the registered debt on any securities exchange. The
                            initial purchasers of the outstanding debt have
                            advised Monsanto that they currently intend to make
                            a

                                       5
<PAGE>

                            market in the registered debt following the
                            exchange offer, but they are not obligated to do
                            so, and any market-making may be stopped at any
                            time without notice. Monsanto does not know if an
                            active public market for the registered notes and
                            debentures will develop or, if developed, will
                            continue. If an active public market does not
                            develop or is not maintained, the market price and
                            liquidity of the registered notes and debentures
                            may be adversely affected. Monsanto cannot make any
                            assurances regarding the liquidity of the market
                            for such registered debt, the ability of holders to
                            sell their registered debt or the price at which
                            holders may sell their registered debt.

   For additional information regarding the registered debt, see "Description
of the Registered Debt."

                                Other Offerings

   In addition to the Monsanto's private placement of the outstanding debt, in
November 1998 Monsanto completed offerings of common stock and adjustable
conversion-rate equity security units that resulted in aggregate net proceeds,
after deducting estimated expenses, of approximately $1.64 billion. This
prospectus relates only to the offering of registered debt and no other
securities.

                                       6
<PAGE>

                                  RISK FACTORS

   Before making a decision to exchange your notes and debentures in the
exchange offer, you should consider, in addition to the information with
respect to Monsanto and its business contained in this prospectus or
incorporated in this prospectus by reference, the following risk factors
relating to the exchange offer.

             Risks Specifically Relating to the Exchange Offer

Failure to Exchange Outstanding Debt for Registered Debt Limits Your Ability to
Sell or Transfer the Notes and Debentures

   Monsanto did not register the outstanding debt under the Securities Act or
any state securities laws, nor does it intend to after the exchange offer. As a
result, the outstanding debt may only be transferred in limited circumstances
under the securities laws. If the holders of the outstanding debt who are
eligible to participate in the exchange offer do not exchange their notes and
debentures in the exchange offer, they lose their right to have the outstanding
debt registered under the Securities Act. A holder of outstanding debt after
the exchange offer may be unable to sell the notes and debentures.

Lack of Public Market for Registered Debt May Limit its Liquidity

   Monsanto does not intend to apply for a listing of the registered debt on
any securities exchange. The initial purchasers of the outstanding debt have
advised Monsanto that they currently intend to make a market in the registered
debt following the exchange offer, but they are not obligated to do so, and
they may stop any market-making at any time without notice. Monsanto does not
know if an active public market for the registered debt will develop or, if
developed, will continue. If an active public market does not develop or is not
maintained, the market price and liquidity of the registered debt may be
adversely affected. Monsanto cannot make any assurances regarding the liquidity
of the market for the registered debt, the ability of holders to sell their
registered debt or the price at which holders may sell their registered debt.

Your Failure to Properly Tender Your Outstanding Debt May Forfeit Your
Opportunity to Exchange it for Registered Debt

   Monsanto will issue registered debt in exchange for the outstanding debt
pursuant to the exchange offer only after it timely receives the outstanding
debt, a properly completed and duly executed letter of transmittal and all
other required documents. Therefore, holders of the outstanding debt desiring
to tender such securities in exchange for registered debt should allow enough
time to ensure timely delivery to the exchange agent. Monsanto has no duty to
give notice of defects or irregularities with respect to the tenders of
outstanding debt for exchange. Outstanding debt that is not tendered or is
tendered but not accepted will, after the consummation of the exchange offer,
continue to be subject to the existing transfer restrictions. In addition, upon
consummation of the exchange offer, certain registration rights will terminate.

   In addition, any holder of outstanding debt who tenders in the exchange
offer for the purpose of participating in a distribution of the registered debt
may be deemed to have received restricted securities and, if so, will be
required to comply with the registration and prospectus delivery requirements
of the Securities Act in connection with any resale transactions. Each
participating broker-dealer that receives registered debt for its own account
in exchange for outstanding debt, where such outstanding debt was acquired by
such participating broker-dealer as a result of market-making activities or
other trading activities, must acknowledge that it will deliver a prospectus in
connection with any resale of the registered debt. See "Plan of Distribution."
To the extent that outstanding debt is tendered and accepted in the exchange
offer, the trading market for untendered and tendered but unaccepted
outstanding debt could be adversely affected.

                                       7
<PAGE>


              Risks Generally Relating to Monsanto's Business

Monsanto's Significant Financial Requirements Could Hinder its Ability to
Innovate, Introduce and Distribute New Products

   Monsanto has committed significant financial resources to finance its recent
acquisitions. In addition, Monsanto requires financial resources to fund the
research, development and product launch of Monsanto's technological
innovations. Monsanto's inability to fund these activities could hinder its
ability to innovate, introduce and distribute new products.

The Value of Monsanto's Intellectual Property Rights Could be Diminished

   Monsanto's patents and trademarks are materially important to the operation
of its business. Monsanto relies on patent and trademark laws worldwide to
protect its intellectual property rights. Even so,

  .  there is uncertainty about the value of patent protection in certain
     countries outside the United States;

  .  competitors may patent technologies before Monsanto and thus render
     Monsanto's technology less valuable;

  .  Monsanto's products may unknowingly rely on others' technology and thus
     require Monsanto to obtain licenses from its competitors; and

  .  some of Monsanto's intellectual property rights are the subject of
     litigation.

Monsanto Derives a Substantial Portion of its Revenues from Foreign Markets
Which Subjects Monsanto to Foreign Economic Risks

   Approximately 45% of Monsanto's 1998 revenues resulted from sales of its
products outside the United States. Declines in these foreign economies could
result in a decline in sales of Monsanto's products overseas if Monsanto's
products were to become too expensive to buy in foreign local currencies.
Monsanto also faces the foreign risk of

  .  changes in currency exchange rates;

  .  changes in political or economic conditions;

  .  trade protection measures;

  .  import or export licensing requirements; and

  .  unexpected changes in regulatory requirements.

Monsanto's Inability to Forge Future Alliances to Co-promote and Co-distribute
its Products Could Limit its Ability to Quickly Commercialize, Distribute and
Market its New Products

   Monsanto partly relies upon strategic alliances and joint ventures through
which Monsanto and third parties co-promote and co-distribute Monsanto's
products. These arrangements help Monsanto speed the development and
commercialization of its new products and assist in their distribution and
marketing. Monsanto cannot guarantee that it will succeed in forging future
alliances or joint ventures with third parties. Monsanto's failure to do so
could limit its ability to quickly commercialize, distribute and market its new
products.

Monsanto's Restructuring Plan Could Interrupt its Business

   Monsanto's plan to restructure its business includes eliminating a number of
employment positions and selling some non-strategic assets. The success of such
restructuring inherently is uncertain. For example, Monsanto may be unable to
successfully negotiate acceptable prices for the sales of its assets. In
addition, such restructuring could temporarily interrupt Monsanto's business or
cause its business to lose momentum.


                                       8
<PAGE>


Monsanto May Face Delays and Increased Costs in Implementing its Year 2000
Readiness Program

   Monsanto cannot guarantee that its estimates for completing its Year 2000
Program will be achieved on time. Monsanto may face delays and increased costs
in implementing this program due to

  .  the availability and cost of experts in this area;

  .  the ability to locate and correct all relevant computer code and
     embedded systems; and

  .  the success of similar programs conducted by suppliers and other third
     parties.

    Risks Specifically Relating to Monsanto's Agricultural Products Business

Generic Competition for Roundup(R) Herbicides Could Affect Sales

   The family of Roundup(R) herbicides is a major product line for Monsanto's
agricultural products business. These herbicides are likely to face increasing
competition from generic products. Foreign patents protecting Roundup(R)
herbicides have expired in most countries and a major U.S. patent expires in
September 2000.

Delays in Winning Government and Consumer Acceptance Could Delay the Commercial
Success of Biotech Products

   The commercial success of Monsanto's biotech agricultural and food products
depends on government and consumer acceptance. Claims that genetically-modified
plant products are unsafe or pose unknown risks may negatively influence public
attitudes about these products. Negative government or public attitudes could
hinder Monsanto's ability to sell these products in foreign markets or the
ability of farmers to sell crops grown from Monsanto's products. As a result,
such factors could delay the commercial success of Monsanto's biotech products.

Competition in Biotechnology Research Could Render Monsanto's Products Less
Competitive

   Several companies already engage in plant biotechnology research. Some of
these companies have greater financial, technical and marketing resources than
Monsanto. Competition may intensify as other companies enter the market. The
ability to be first to market a new product can create a significant
competitive advantage. As a result, technological advances by others could
render Monsanto's products less competitive.

Monsanto's Integration of Recent Acquisitions will be Dilutive to its Financial
Results and Monsanto's Failure to Successfully Integrate these Acquisitions
Could Further Affect its Financial Results

   Monsanto's recent acquisitions of DEKALB Genetics Corp., Plant Breeding
International Cambridge and specific international seed operations of Cargill
Inc. will significantly dilute Monsanto's financial results in the short term.
In the long term, Monsanto must integrate these acquired companies into its
business if it is to realize projected synergies and provide the distribution
channels needed to quickly and efficiently launch new products. This process of
integration could temporarily interrupt Monsanto's business or cause it to lose
momentum.

Seasonality of the Agricultural Products Business Impacts Financial Results

   Monsanto's agricultural products business is highly seasonal. It is subject
to weather conditions and natural disasters that affect commodity prices, seed
yields, and decisions by growers regarding purchases of seed and herbicides.
Commodity prices for several crops have decreased significantly. These lower
commodity prices affect growers' decisions about the types and amounts of crops
to plant and may negatively influence sales of Monsanto's herbicide and seed
products. Monsanto can give no assurance that this trend will not continue.

                                       9
<PAGE>

       Risks Specifically Relating to Monsanto's Pharmaceuticals Business

Pharmaceuticals Businesses Have Many Inherent Risks Which Could Delay
Monsanto's Commercialization of New Products

   Pharmaceutical research and development is subject to inherent uncertainty,
difficulties and delays. These include, but are not limited to, successful
completion of clinical trials and the ability to obtain regulatory approval for
the compounds worldwide. Monsanto's failure to receive government approvals as
anticipated could preclude or substantially delay commercialization of products
currently in research and development programs.

Monsanto's Inability to Commercialize New Products and Expand the Use of Old
Products Could Diminish the Success of its Pharmaceuticals Business

   The long-term success of Monsanto's pharmaceuticals business largely depends
on Monsanto's ability to commercialize new products and expand the use of
existing products. This involves substantial funding of research and
development programs and incurring new product launch expenses. Monsanto's
failure to fund such expenses could limit its ability to commercialize new
products or expand the use of existing products. Monsanto cannot guarantee that
anticipated product test results will materialize or that newly-marketed
pharmaceutical products will be commercially successful.

Pharmaceuticals Businesses May Face Potential Product Liability Lawsuits and
Associated Adverse Publicity Which Could Lead to Recalls, Withdrawals or
Declining Sales for Monsanto's Pharmaceuticals Products

   The sale of pharmaceutical products always involves a risk of product
liability claims and associated adverse publicity. Substantial damage awards
for injuries allegedly caused by the use of pharmaceutical products have been
made against other companies in past years. In addition, unexpected safety or
efficacy concerns can arise with respect to marketed products. Whether or not
they are scientifically justified, such concerns could lead to recalls,
withdrawals, or declining sales for Monsanto's pharmaceuticals products.

Competition in Monsanto's Pharmaceuticals Product Research Could Reduce Sales

   Many of Monsanto's pharmaceutical competitors have greater research,
financial, marketing and other resources than Monsanto. Some of Monsanto's
trademarked pharmaceutical products also face increasing pressures from
producers of lower-priced generic products and from new products entering the
marketplace. As Monsanto introduces new products intended for use in the
treatment of the same conditions as existing Monsanto products, sales of such
existing Monsanto products may suffer.

Downward Pricing Pressures Could Affect the Profitability of Pharmaceutical
Products

   Managed care groups, health care organizations and government agencies
worldwide actively seek discounts and lower prices on pharmaceutical products.
Monsanto may be unable to negotiate prices for its products which
simultaneously provide overall economic benefits to health care providers and
allow Monsanto to maintain profits at acceptable levels.


                                       10
<PAGE>

                                USE OF PROCEEDS

   Monsanto will not receive any proceeds from the exchange offer. In
consideration for issuing the registered debt, Monsanto will receive in
exchange outstanding debt of like principal amount, the terms of which are
identical in all material respects to the registered debt. The outstanding debt
surrendered in exchange for registered debt will be retired and canceled and
cannot be reissued. Accordingly, issuance of the registered debt will not
result in any increase in Monsanto's indebtedness. Monsanto has agreed to bear
the expenses of the exchange offer. No underwriter is being used in connection
with the exchange offer.

   The net proceeds to Monsanto from the sale of the outstanding debt, after
deducting expenses, was approximately $2.475 billion. Monsanto used such net
proceeds to finance or refinance seed company acquisitions, including to
refinance its outstanding commercial paper as it became due, and for working
capital purposes.

                                       11
<PAGE>

                               THE EXCHANGE OFFER

Purpose of the Exchange Offer

   Simultaneously with the sale of the outstanding debt, Monsanto entered into
a registration rights agreement with Salomon Smith Barney Inc. and Goldman,
Sachs & Co., the initial purchasers of the outstanding debt. Under this
registration rights agreement, Monsanto agreed to file a registration statement
regarding the exchange of the outstanding debt for registered notes and
debentures with terms identical in all material respects. Monsanto also agreed
to use its reasonable best efforts to cause that registration statement to
become effective with the Securities and Exchange Commission. A copy of the
registration rights agreement has been filed as an exhibit to the registration
statement of which this prospectus is a part.

   Monsanto is conducting the exchange offer to satisfy its contractual
obligations under the registration rights agreement. The form and terms of the
registered debt are the same as the form and terms of the outstanding debt,
except that the registered debt will be registered under the Securities Act,
and holders of the registered debt will not be entitled to liquidated damages.
The registration rights agreement requires Monsanto to pay liquidated damages
to holders of the outstanding debt from June 8, 1999 until it consummates the
exchange offer. See "Description of the Registered Debt--Registration Rights;
Liquidated Damages" for more information on liquidated damages. Upon the
completion of the exchange offer, holders of outstanding debt will not be
entitled to any liquidated damages on the outstanding debt or any further
registration rights under the registration rights agreement, except under
limited circumstances. The exchange offer is not extended to outstanding debt
holders in any jurisdiction where the exchange offer does not comply with the
securities or blue sky laws of that jurisdiction.

   In the event that (1) applicable interpretations of the SEC's staff do not
permit Monsanto to conduct the exchange offer, or (2) certain holders of the
outstanding debt notify Monsanto that they are not eligible to participate in,
or would not receive freely tradeable registered debt in exchange for tendered
outstanding debt in, the exchange offer, Monsanto will use its reasonable
efforts to cause to become effective a shelf registration statement with
respect to the resale of the outstanding debt.

   Monsanto also agreed to use its reasonable efforts to keep the shelf
registration statement effective until the earlier of (1) two years after the
date of the issuance of the outstanding debt, unless Rule 144(k) is amended to
provide a shorter restrictive period, or (2) such time as when all the
securities covered by the shelf registration statement have been sold.

   The term "holder" as used in this section of the prospectus entitled "The
Exchange Offer" means (1) any person in whose name the outstanding debt is
registered on Monsanto's books, or (2) any other person who has obtained a
properly completed bond power from the registered holder, or (3) any person
whose outstanding debt is held of record by qDTC and who wants to deliver such
outstanding debt by book-entry transfer at DTC.

Terms of the Exchange Offer

   Monsanto is offering to exchange up to $2,500,000,000 total principal amount
of registered debt for a like total principal amount of outstanding debt. The
outstanding debt must be tendered properly on or before the expiration date of
the exchange offer and not withdrawn. In exchange for outstanding debt properly
tendered and accepted, Monsanto will issue a like total principal amount of up
to $2,500,000,000 in registered debt.

   The exchange offer is not conditioned upon holders tendering a minimum
principal amount of outstanding debt. As of the date of this prospectus,
$2,500,000,000 aggregate principal amount of outstanding debt is outstanding.

                                       12
<PAGE>

   Holders of the outstanding debt do not have any appraisal or dissenters'
rights in the exchange offer. If holders do not tender outstanding debt or
tender outstanding debt that Monsanto does not accept, their outstanding debt
will remain outstanding. Any outstanding debt will be entitled to the benefits
of the indenture, but will not be entitled to any further registration rights
under the registration rights agreement, except under limited circumstances.

   After the expiration date of the exchange offer, Monsanto will return to the
holder any tendered outstanding debt that Monsanto did not accept for exchange
due to, among other things, an invalid tender.

   Holders exchanging outstanding debt will not have to pay brokerage
commissions or fees or transfer taxes if they follow the instructions in the
letter of transmittal. Monsanto will pay the charges and expenses, other than
certain taxes described below, in the exchange offer. See "--Fees and Expenses"
for further information regarding fees and expenses.

   Neither Monsanto nor Monsanto's board of directors recommends that you
tender or not tender your outstanding debt in the exchange offer. In addition,
Monsanto has not authorized anyone to make any recommendation. You must decide
whether to tender your outstanding debt in the exchange offer and, if so, the
aggregate amount of such outstanding debt to tender.

   As used in this prospectus, the "expiration date" of the exchange offer is
5:00 p.m., New York City time, on       , 2000, unless Monsanto, in its sole
discretion, extends the exchange offer to allow additional tenders of
outstanding debt. The registration rights agreement requires Monsanto to keep
the exchange offer open for at least thirty days. Because Monsanto did not
consummate the exchange offer by July 22, 1999, the registration rights
agreement requires Monsanto to pay liquidated damages to the holders of
outstanding debt until it consummates the exchange offer. Consequently,
Monsanto does not expect to extend the exchange offer past the expiration date
but may do so to permit additional holders to exchange their outstanding debt.

   Monsanto has the right, in accordance with applicable law, at any time:

  .  to delay the acceptance of the outstanding debt;

  .  to terminate the exchange offer if Monsanto determines that any of the
     conditions to the exchange offer have not occurred or have not been
     satisfied;

  .  to extend the expiration date of the exchange offer to, among other
     things, update its disclosure and keep all outstanding debt tendered
     other than those outstanding notes and debentures properly withdrawn;
     and

  .  to waive any condition or amend the terms of the exchange offer.

   If Monsanto materially changes the exchange offer, or if Monsanto waives a
material condition of the exchange offer, Monsanto will promptly distribute a
prospectus supplement to the holders of the outstanding debt disclosing the
change or waiver. Monsanto also will extend the exchange offer as required by
Rule 14e-1 under the Securities Exchange Act of 1934.

   If Monsanto exercises any of the rights listed above, it will promptly give
oral or written notice of the action to the exchange agent and will issue a
release to an appropriate news agency. In the case of an extension, an
announcement will be made no later than 9:00 a.m., New York City time, on the
next business day after the previously scheduled expiration date.

Acceptance for Exchange and Issuance of Registered Debt

   Monsanto will issue registered debt to the exchange agent for outstanding
debt tendered and accepted, and not withdrawn, promptly on or after the
expiration date. The exchange agent might not deliver the registered debt to
all tendering holders at the same time. The timing of delivery depends upon
when the exchange agent receives and processes the required documents.

                                       13
<PAGE>

   Monsanto will be deemed to have exchanged outstanding debt validly tendered
and not withdrawn when Monsanto gives oral or written notice to the exchange
agent of their acceptance. The exchange agent is an agent for Monsanto for
purposes of receiving tenders of outstanding debt, letters of transmittal and
related documents. The exchange agent also is an agent for tendering holders
for purposes of receiving outstanding debt, letters of transmittal and related
documents and transmitting registered debt to validly tendering holders. If for
any reason, Monsanto (1) delays the acceptance or exchange of any outstanding
debt; (2) extends the exchange offer; or (3) is unable to accept or exchange
outstanding debt, then the exchange agent may, on behalf of Monsanto, and
subject to Rule 14e-1(c) under the Exchange Act, retain tendered notes and
debentures. Tendered notes and debentures retained by the exchange agent may
not be withdrawn, except according to the withdrawal procedures outlined in the
section entitled "--Withdrawal Rights" below.

   In tendering outstanding debt, you must warrant in the letter of transmittal
or in an "Agent's Message", as described below, that:

  .  you have full power and authority to tender, exchange, sell, assign and
     transfer your outstanding debt;

  .  Monsanto will acquire good, marketable and unencumbered title to the
     tendered outstanding debt, free and clear of all liens, restrictions,
     charges and other encumbrances; and

  .  the outstanding debt tendered for exchange is not subject to any adverse
     claims or proxies.

   You also must warrant and agree that you will, upon request, execute and
deliver any additional documents requested by Monsanto or the exchange agent to
complete the exchange, sale, assignment, and transfer of the outstanding debt
and that you will comply with your obligations under the registration rights
agreement.

Procedures for Tendering Outstanding Debt

 Valid Tender

   You may tender your outstanding debt by book-entry transfer or, if you hold
certificated securities, by other means, as described below. For book-entry
transfer, you must deliver to the exchange agent either (1) a completed and
signed letter of transmittal or (2) an "agent's message". An agent's message
means a message, transmitted by DTC to and received by the exchange agent which
forms part of a confirmation of a book-entry transfer of outstanding debt into
the exchange agent's account at DTC. The agent's message states that DTC has
received an express acknowledgment from the tendering participant that such
participant has received and agrees to be bound by the letter of transmittal
and that Monsanto may enforce the letter of transmittal against such
participant.

   You must deliver your letter of transmittal or the agent's message by mail,
facsimile, hand delivery or overnight carrier to the exchange agent on or
before the expiration date. In addition, to complete a book-entry transfer, you
must also either (1) have DTC transfer the outstanding debt into the exchange
agent's account at DTC using the ATOP procedures for transfer, and obtain a
confirmation of such transfer, or (2) follow the guaranteed delivery procedures
described below under "--Guaranteed Delivery Procedures."

   If you tender less than all of your outstanding debt, you should fill in the
principal amount of notes and debentures tendered in the appropriate box on the
letter of transmittal. If you do not indicate the amount tendered in the
appropriate box, Monsanto will assume you are tendering all outstanding debt
that you hold.

   For tendering your outstanding debt other than by book-entry transfer, you
must deliver a completed and signed letter of transmittal to the exchange
agent. Again, you must deliver the letter of transmittal by mail, facsimile,
hand delivery or overnight carrier to the exchange agent on or before the
expiration date. In addition, to complete a valid tender you must either (1)
deliver your outstanding debt to the exchange agent on or before the expiration
date, or (2) follow the guaranteed delivery procedures set forth below under
"--Guaranteed Delivery Procedures."

                                       14
<PAGE>

   Delivery of required documents by whatever method you choose is at your sole
risk. Delivery is complete when the exchange agent actually receives the items
to be delivered. Delivery of documents to DTC in accordance with DTC's
procedures does not constitute delivery of such documents to the exchange
agent. If delivery is made by mail, then registered mail, return receipt
requested, properly insured, or an overnight delivery service is recommended.
In all cases, you should allow sufficient time to ensure timely delivery.

 Signature Guarantees

   You do not need to endorse certificates for the outstanding debt or provide
signature guarantees on the letter of transmittal, unless (1) someone other
than the registered holder tenders the certificate or (2) you complete the box
entitled "Special Issuance Instructions" or "Special Delivery Instructions" in
the letter of transmittal. In the case of (1) or (2) above, you must sign your
outstanding debt or provide a properly executed bond power, with the signature
on the bond power and on the letter of transmittal guaranteed by a firm or
other entity identified in Rule 17Ad-15 under the Exchange Act as an "eligible
guarantor institution."

   An "eligible guarantor institution" includes: (1) a bank; (2) a broker,
dealer, municipal securities broker or dealer or government securities broker
or dealer; (3) a credit union; (4) a national securities exchange, registered
securities association or clearing agency; or (5) a savings association that is
a participant in a securities transfer association.

 Guaranteed Delivery

   If you want to tender your outstanding debt in the exchange offer and (1)
the certificates for the outstanding debt are not immediately available or all
required documents are unlikely to reach the exchange agent on or before the
expiration date, or (2) a book-entry transfer cannot be completed in time, you
may tender your outstanding debt if you comply with the following guaranteed
delivery procedures:

     (a) you tender through an eligible guarantor institution;

     (b) you deliver a properly completed and signed notice of guaranteed
  delivery, like the form provided with the letter of transmittal, to the
  exchange agent on or before the expiration date; and

     (c) you deliver the certificates or a confirmation of book-entry
  transfer and a properly completed and signed letter of transmittal to the
  exchange agent within three New York Stock Exchange trading days after the
  notice of guaranteed delivery is executed.

   You may deliver the notice of guaranteed delivery by hand, facsimile or mail
to the exchange agent and must include a guarantee by an eligible guarantor
institution in the form described in the notice.

   Monsanto's acceptance of properly tendered outstanding debt is a binding
agreement between the tendering holder and Monsanto upon the terms and subject
to the conditions of the exchange offer.

 Determination of Validity

   Monsanto will resolve all questions regarding the form of documents,
validity, eligibility (including time of receipt) and acceptance for exchange
of any tendered outstanding debt. Monsanto's resolution of these questions and
Monsanto's interpretation of the terms and conditions of the exchange offer,
including the letter of transmittal, are final and binding on all parties. A
tender of outstanding debt is invalid until all irregularities have been cured
or waived. None of Monsanto, any of its affiliates or assigns, the exchange
agent nor any other person is under any obligation to give notice of any
irregularities in tenders nor will they be liable for failing to give any such
notice. Monsanto reserves the absolute right, in its sole and absolute
discretion, to reject any tenders determined to be in improper form or
unlawful. Monsanto also reserves the absolute right to waive any of the
conditions of the exchange offer or any condition or irregularity in the tender
of outstanding debt by any holder. Monsanto need not waive similar conditions
or irregularities in the case of other holders.

                                       15
<PAGE>

   If any letter of transmittal, endorsement, bond power, power of attorney, or
any other document required by the letter of transmittal is signed by a
trustee, executor, administrator, guardian, attorney-in-fact, officer of a
corporation or other person acting in a fiduciary or representative capacity,
that person must indicate that capacity when signing. In addition, unless
waived by Monsanto, the person must submit proper evidence satisfactory to
Monsanto, in its sole discretion, of his or her authority to so act.

   A beneficial owner of outstanding debt that is held by or registered in the
name of a broker, dealer, commercial bank, trust company or other nominee or
custodian should contact that entity promptly if the holder wants to
participate in the exchange offer.

Resales of Registered Debt

   Monsanto is exchanging the outstanding debt for registered debt based upon
the position of the SEC's staff, set forth in interpretive letters to third
parties in other similar transactions. Monsanto will not seek its own
interpretive letter. As a result, Monsanto cannot assure you that the SEC's
staff will take the same position on this exchange offer as it did in
interpretive letters to other parties. Based on the SEC's staff's letters to
other parties, Monsanto believes that holders of registered debt, other than
broker-dealers, can offer the registered notes and debentures for resale,
resell and otherwise transfer such registered debt without delivering a
prospectus to prospective purchasers, other than as described below.

   Any holder of outstanding debt who is an "affiliate" of Monsanto or who
intends to distribute registered debt, or any broker-dealer who purchased
outstanding debt from Monsanto for resale pursuant to Rule 144A or any other
available exemption under the Securities Act:

  .  cannot rely on the SEC's staff's interpretations in the above-mentioned
     interpretive letters;

  .  cannot tender outstanding debt in the exchange offer; and

  .  must comply with the registration and prospectus delivery requirements
     of the Securities Act to transfer the outstanding debt, unless the sale
     is exempt.

   In addition, if any broker-dealer acquired outstanding debt for its own
account as a result of market-making or other trading activities and exchanges
such outstanding debt for registered debt, such broker-dealer must deliver a
prospectus with any resales of its registered debt.

   If you want to exchange your outstanding debt for registered debt, you will
be required to affirm that:

  .  you are not an "affiliate" of Monsanto;

  .  you are acquiring the registered debt in the ordinary course of your
     business;

  .  you have no arrangement or understanding with any person to participate
     in a "distribution", within the meaning of the Securities Act, of the
     registered debt; and

  .  if you are not a broker-dealer, you are not engaged in, and do not
     intend to engage in, a "distribution", within the meaning of the
     Securities Act, of the registered debt.

   In addition, Monsanto may require you to provide information regarding the
number of "beneficial owners," within the meaning of Rule 13d-3 under the
Exchange Act, of the outstanding debt. Each broker-dealer that receives
registered debt for its own account must acknowledge that it acquired its
outstanding debt for its own account as the result of market-making activities
or other trading activities and must agree that it will deliver a prospectus
meeting the requirements of the Securities Act in connection with any resale of
such registered debt. By making this acknowledgment and by delivering a
prospectus, a broker-dealer will not be deemed to admit that it is an
"underwriter" under the Securities Act. Based on the SEC's staff's position in
certain interpretive letters, Monsanto believes that broker-dealers who
acquired outstanding debt for their own accounts as a result of market-making
activities or other trading activities may fulfill their prospectus delivery

                                       16
<PAGE>

requirements with respect to the registered debt with a prospectus meeting the
requirements of the Securities Act. Accordingly, a broker-dealer may use this
prospectus to satisfy such requirements. Monsanto has agreed that, starting on
the expiration date of the exchange offer and ending on the close of business
on the 180th day following such expiration date, it will make this prospectus,
as amended or supplemented, available to any broker-dealer for use in
connection with any such resale. See "Plan of Distribution" for more
information. A broker-dealer intending to use this prospectus in the resale of
registered debt must notify Monsanto, on or prior to the expiration date, that
it is a participating broker-dealer. This notice may be given in the letter of
transmittal or may be delivered to the exchange agent. Any participating
broker-dealer who is an "affiliate" of Monsanto may not rely on the SEC's
staff's interpretive letters and must comply with the registration and
prospectus delivery requirements of the Securities Act when reselling its
registered debt.

   Monsanto agrees to advise holders of, among other things:

  .  any SEC request for amendments or supplements to the registration
     statement or this prospectus or for additional information;

  .  the SEC's issuance of any stop order suspending the effectiveness of the
     registration statement or the initiation of any proceedings for that
     purpose; and

  .  Monsanto's receipt of any notification with respect to the suspension of
     the qualification of the registered debt in any jurisdiction or the
     initiation or threatening of any proceeding for such purpose.

   Upon the occurrence of any of these events, Monsanto agrees to notify the
holders, if applicable, to suspend use of this prospectus and Monsanto shall
prepare, using its reasonable efforts to do so as soon as possible, a post-
effective amendment to the registration statement or an amendment or supplement
to this prospectus or file any other required document so that, as thereafter
delivered to purchasers of the registered debt, the prospectus will not include
an untrue statement of a material fact or omit to state any material fact
necessary to make the statements herein, in the light of the circumstances
under which they were made, not misleading. The holders, if applicable, agree
that they shall suspend use of this prospectus until Monsanto has amended or
supplemented the prospectus so that it does not contain any such untrue
statement or omission.

Withdrawal Rights

   You can withdraw tenders of outstanding debt at any time on or before the
expiration date of the exchange offer.

   For a withdrawal to be effective, you must deliver a written or facsimile
transmission of a "notice of withdrawal" to the exchange agent on or before the
expiration date. The notice of withdrawal must specify the name of the person
tendering the outstanding debt to be withdrawn, the total principal amount of
outstanding debt withdrawn, and the name of the registered holder of the
outstanding debt if it is different from the name of the person tendering the
outstanding debt. If you delivered outstanding debt to the exchange agent, you
must submit the serial numbers of the outstanding debt to be withdrawn and the
signature on the notice of withdrawal must be guaranteed by an eligible
guarantor institution, except in the case of outstanding debt tendered for the
account of an eligible guarantor institution. If you tendered outstanding debt
as a book-entry transfer, the notice of withdrawal must specify the name and
number of the account at DTC to be credited with the withdrawal of outstanding
debt and you must deliver the notice of withdrawal to the exchange agent by
written, telegraphic, telex or facsimile transmission. You may not rescind
withdrawals of tender. Outstanding debt properly withdrawn may again be
tendered at any time on or before the expiration date.

   Monsanto will determine all questions regarding the validity, form and
eligibility of withdrawal notices. Monsanto's determination will be final and
binding on all parties. None of Monsanto, any of its affiliates or assigns, the
exchange agent nor any other person is under any obligation to give notice of
any irregularities in any notice of withdrawal, nor will they be liable for
failing to give any such notice. Withdrawn outstanding debt will be returned to
the holder after withdrawal.

                                       17
<PAGE>

Interest on Registered Debt

   The registered debt will bear interest at the respective rates per year that
appear on the cover page of this prospectus, payable semi-annually, on June 1
and December 1 of each year, commencing June 1, 2000. Holders of registered
debt will receive interest on June 1, 2000 from the date of initial issuance of
the registered debt, plus an amount equal to the accrued, but unpaid, interest
on the outstanding debt. Interest on the outstanding debt accepted for exchange
will cease to accrue upon issuance of the registered debt.

Conditions to the Exchange Offer

   Monsanto need not exchange any outstanding debt, may terminate the exchange
offer or may waive any conditions to the exchange offer or amend the exchange
offer, if any of the following conditions have occurred:

  .  the SEC's staff no longer allows the registered debt to be offered for
     resale, resold and otherwise transferred by certain holders without
     compliance with the registration and prospectus delivery provisions of
     the Securities Act;

  .  a governmental body passes any law, statute, rule or regulation which,
     in Monsanto's opinion, prohibits or prevents the exchange offer;

  .  the SEC or any state securities authority issues a stop order suspending
     the effectiveness of the registration statement or initiates or
     threatens to initiate a proceeding to suspend the effectiveness of the
     registration statement; or

  .  Monsanto is unable to obtain any governmental approval that Monsanto
     believes is necessary to complete the exchange offer.

   If Monsanto reasonably believes that any of the above conditions has
occurred, it may (1) terminate the exchange offer, whether or not any
outstanding debt has been accepted for exchange, (2) waive any condition to the
exchange offer or (3) amend the terms of the exchange offer in any respect.
Monsanto's failure at any time to exercise any of these rights will not waive
such rights, and each right will be deemed an ongoing right which may be
asserted at any time or from time to time. However, Monsanto does not intend to
terminate the exchange offer if none of the preceding conditions has occurred.

Exchange Agent

   Monsanto appointed The Bank of New York as exchange agent for the exchange
offer. Holders should direct questions and requests for assistance, requests
for additional copies of this prospectus or of the letter of transmittal and
requests for notice of guaranteed delivery to the exchange agent addressed as
follows:

    By Registered or Certified Mail:         By Hand Or Overnight Delivery:
          The Bank of New York                    The Bank of New York
                                                   101 Barclay Street
        101 Barclay Street
        New York, New York 10286            Corporate Trust Services Window
                                                      Ground Level
             REORG-7E
                                                New York, New York 10286
   Attention: Carolle Montreuil
                                           Attention: Carolle Montreuil

                             Confirm by Telephone:

                             Carolle Montreuil

                              (212) 815-5788

                            Facsimile Transmissions:
                     (Eligible Guarantor Institutions Only)

                              (212) 815-6339

   If you deliver letters of transmittal and any other required documents to an
address or facsimile number other than those listed above, your tender is
invalid.

                                       18
<PAGE>

Fees and Expenses

   Monsanto will pay the exchange agent reasonable and customary fees for its
services and reasonable out-of-pocket expenses. Monsanto will also pay
brokerage houses and other custodians, nominees and fiduciaries their
reasonable out-of-pocket expenses for sending copies of this prospectus and
related documents to holders of outstanding debt, and in handling or tendering
for their customers.

   Monsanto will pay the transfer taxes for the exchange of the outstanding
debt in the exchange offer. If, however, exchange debt is delivered to or
issued in the name of a person other than the registered holder, or if a
transfer tax is imposed for any reason other than for the exchange of
outstanding debt in the exchange offer, then the tendering holder will pay the
transfer taxes. If a tendering holder does not submit satisfactory evidence of
payment of taxes or exemption from taxes with the letter of transmittal, the
taxes will be billed directly to the tendering holder.

   Monsanto will not make any payment to brokers, dealers or other nominees
soliciting acceptances in the exchange offer.

Accounting Treatment

   The registered debt will be recorded at the same carrying value as the
outstanding debt. Accordingly, Monsanto will not recognize any gain or loss for
accounting purposes. Monsanto intends to amortize the expenses of the exchange
offer and issuance of the outstanding debt over the respective terms of each
series of the registered debt.

                                       19
<PAGE>

                       DESCRIPTION OF THE REGISTERED DEBT

   The outstanding debt was, and the registered debt will be, issued pursuant
to an indenture, dated as of December 1, 1998, between Monsanto and The Bank of
New York, as trustee. A copy of the indenture has been filed as an exhibit to
Monsanto's Current Report on Form 8-K filed with the SEC on December 14, 1998
and is incorporated by reference as an exhibit to the registration statement of
which this prospectus is a part. The terms of the registered debt are the same
as the terms of the outstanding debt, except that (1) Monsanto registered the
registered debt under the Securities Act, and, unlike the outstanding debt, its
transfer is not restricted and (2) holders of the registered debt are not
entitled to certain rights under the registration rights agreement.

   Because this section of the prospectus merely summarizes the terms of the
registered debt, you should read the indenture and the relevant portions of the
Trust Indenture Act of 1939 for more complete information regarding the terms
of the registered debt, including the definition of certain terms used in this
section. Copies of the indenture and registration rights agreement can be
obtained by following the instructions contained in this prospectus under the
headings "Where You Can Find More Information" and "Incorporation of Certain
Documents by Reference."

General

   The registered debt will have the following terms:

<TABLE>
<CAPTION>
                                           Principal   Interest
                                             Amount      Rate    Maturity Date
                                          ------------ -------- ----------------
      <S>                                 <C>          <C>      <C>
      Notes due 2001..................... $500,000,000  5.375%  December 1, 2001
      Notes due 2005..................... $600,000,000  5.750%  December 1, 2005
      Notes due 2008..................... $200,000,000  5.875%  December 1, 2008
      Debentures due 2018................ $500,000,000  6.500%  December 1, 2018
      Debentures due 2028................ $700,000,000  6.600%  December 1, 2028
</TABLE>

   The registered debt consists of five separate series for purposes of the
indenture. The registered debt will be Monsanto's senior unsecured obligations.
Payment of the principal of and interest on the registered debt will rank
equally with all other senior unsecured debt of Monsanto. Certain series of the
registered debt may be redeemable in whole or in part at any time at Monsanto's
option. See "--Optional Redemption." The registered debt will not be entitled
to the benefit of any mandatory redemption or sinking fund. The indenture does
not limit the amount of additional indebtedness Monsanto or any of its
subsidiaries may incur. The indenture does not limit the amount of debt
securities that Monsanto may issue thereunder and provides that such debt
securities may be issued from time to time in one or more series. As of the
date of this prospectus, no debt securities, other than the outstanding debt
described in this prospectus, were outstanding under the indenture.

   The registered debt will bear interest at the respective rates per year that
appear on the cover page of this prospectus and such interest will be payable
semiannually in arrears on June 1 and December 1 of each year, commencing on
June 1, 2000, to the persons in whose names the registered debt is registered
at the close of business on the immediately preceding May 15 and November 15,
respectively. Holders of registered debt will receive interest on June 1, 2000
from the date of initial issuance of the registered debt, plus an amount equal
to the accrued, but unpaid, interest on the outstanding debt. Interest on the
registered debt will accrue from the most recent date to which interest has
been paid or, if no interest has been paid, from the date of original issuance.
Principal of, premium, if any, and interest on the registered debt will be
payable, and the transfer of registered debt will be registrable, at the office
or agency of Monsanto to be maintained for such purpose in the Borough of
Manhattan, The City of New York, except that, at Monsanto's option, interest
may be paid by mailing a check to the address of the person entitled thereto as
it appears on the registered debt register.

                                       20
<PAGE>

   Interest on the registered debt will be computed on the basis of a 360-day
year comprised of twelve 30-day months. The amount of interest payable for any
period shorter than a full semiannual period for which interest is computed
will be computed on the basis of the actual number of days elapsed per 30-day
month. In the event that any date on which principal, premium, if any, or
interest is payable on the registered debt is not a business day (as defined in
the indenture), then payment of the principal, premium, if any, or interest
payable on such date will be made on the next succeeding day that is a business
day. Such payment will be made without any interest or other payment in respect
of any such delay. The registered debt will be issued initially in minimum
denominations of $100,000 and will be available for purchase in integral
multiples of $1,000 in excess thereof.

Optional Redemption

 Redemption Provisions.

   The Notes due 2001 are not redeemable prior to maturity. The other series of
registered debt will be redeemable, as a whole or in part, at Monsanto's
option, at any time or from time to time, on at least 30 days, but not more
than 60 days, prior notice mailed to the registered address of each holder of
registered debt. The redemption prices will be equal to the greater of (1) 100%
of the principal amount of the registered debt to be redeemed and (2) the sum
of the present values of the Remaining Scheduled Payments (as defined below)
discounted, on a semiannual basis which assumes a 360-day year consisting of
twelve 30-day months, at a rate equal to the sum of the applicable Treasury
Rate (as defined below) plus (a) 15 basis points for the Notes due 2005, (b) 20
basis points for the Notes due 2008 and the Debentures due 2018 and (c) 25
basis points for the Debentures due 2028, plus, in each case, accrued interest
to the date of redemption.

   On and after the redemption date, interest will cease to accrue on the
registered debt or any portion of the registered debt called for redemption
(unless Monsanto defaults in the payment of the redemption price and accrued
interest). On or before the redemption date, Monsanto will deposit with a
paying agent (or the trustee) money sufficient to pay the redemption price of
and accrued interest on the registered debt to be redeemed on such date. If
less than all of the registered debt of any series are to be redeemed, the
registered debt to be redeemed shall be selected by the trustee by such method
as the trustee shall deem fair and appropriate.

 Certain Definitions.

   "Treasury Rate" means, with respect to any redemption date, the rate per
annum equal to the semiannual equivalent yield to maturity (computed as of the
second business day immediately preceding such redemption date) of the
Comparable Treasury Issue, assuming a price for the Comparable Treasury Issue
(expressed as a percentage of its principal amount) equal to the Comparable
Treasury Price for such redemption date.

   "Comparable Treasury Issue" means the United States Treasury security
selected by an Independent Investment Banker as having a maturity comparable to
the remaining term of the Notes due 2005, the Notes due 2008, the Debentures
due 2018 or the Debentures due 2028, as the case may be, to be redeemed that
would be utilized, at the time of selection and in accordance with customary
financial practice, in pricing new issues of corporate debt securities of
comparable maturity to the remaining term of such series of registered debt.
"Independent Investment Banker" means one of the Reference Treasury Dealers
appointed by Monsanto.

   "Comparable Treasury Price" means, with respect to any redemption date, the
average of the Reference Treasury Dealer Quotations for such redemption date.
"Reference Treasury Dealer Quotations" means, with respect to each Reference
Treasury Dealer and any redemption date, the average, as determined by the
trustee, of the bid and asked prices for the Comparable Treasury Issue
(expressed in each case as a percentage of its principal amount) quoted in
writing to the trustee by such Reference Treasury Dealer at 3:30 p.m., New York
City time, on the third business day preceding such redemption date.

   "Reference Treasury Dealer" means each of Salomon Smith Barney Inc. and
Goldman, Sachs & Co. and their respective successors. If any of the foregoing
shall cease to be a primary U.S. Government securities dealer (a "Primary
Treasury Dealer"), Monsanto shall substitute another nationally recognized
investment banking firm that is a Primary Treasury Dealer.

                                       21
<PAGE>

   "Remaining Scheduled Payments" means, with respect to registered debt to be
redeemed, the remaining scheduled payments of principal of and interest on such
registered debt that would be due after the related redemption date but for
such redemption. If such redemption date is not an interest payment date with
respect to such registered debt, the amount of the next succeeding scheduled
interest payment on such registered debt will be reduced by the amount of
interest accrued on such registered debt to such redemption date.

Certain Covenants

   Restriction on Liens. The indenture provides that Monsanto will not, nor
will it permit a Restricted Subsidiary to, secure indebtedness for money
borrowed by placing a lien on any Operating Property owned or leased by
Monsanto or any Restricted Subsidiary or on any shares of stock or debt of any
Restricted Subsidiary without equally and ratably securing the registered debt,
unless (1) the principal amount of such indebtedness plus (2) the Attributable
Debt in respect of Sale and Leaseback Transactions (other than Sale and
Leaseback Transactions the proceeds of which are applied as provided under the
second bullet point of the following paragraph) does not exceed 10% of the
Consolidated Net Assets of Monsanto and its consolidated subsidiaries.

   This restriction will not apply to, and there shall be excluded in computing
secured indebtedness for purposes of this restriction, certain permitted liens,
including:

  .  liens existing as of the date of the indenture;

  .  liens existing at the time any corporation becomes a Restricted
     Subsidiary;

  .  liens on property existing at the time of acquisition and certain
     purchase money or similar liens;

  .  liens to secure certain development, operation, construction,
     alteration, repair or improvement costs;

  .  liens securing indebtedness owing to Monsanto or another Restricted
     Subsidiary by a Restricted Subsidiary;

  .  liens in connection with either government contracts, including the
     assignment of moneys due or to become due thereon or obligations issued
     by a state or a commonwealth or certain other governmental entities;

  .  certain liens in connection with legal proceedings or arising in the
     ordinary course of business and not in connection with the borrowing of
     money; and

  .  extensions, substitutions, replacements or renewals of the foregoing.
     (Section 1007)

   Restriction on Sale and Leaseback Transactions. The indenture further
provides that Monsanto will not, nor will it permit any Restricted Subsidiary
to, enter into any Sale and Leaseback Transaction unless either

   (1) the Attributable Debt in respect thereto and all other sale and
leaseback transactions entered into after the date of the indenture (other than
those the proceeds of which are applied to reduce indebtedness under (2)
following), plus the aggregate amount of then outstanding secured indebtedness
not otherwise permitted or excepted without equally and ratably securing the
registered debt, does not exceed 10% of the Consolidated Net Assets of Monsanto
and its consolidated subsidiaries, or

   (2) an amount equal to the fair value of the Operating Property leased is
applied within 120 days to (a) the purchase of any asset or any interest in an
asset which would qualify after purchase, as an Operating Property or (b) the
retirement of the registered debt or other indebtedness maturing more than one
year thereafter. (Section 1008)

 Certain Definitions.

   "Attributable Debt", in respect of the sale and leaseback transactions
described above, means, as of the time of determination, the total obligation
(discounted to present value at the rate per year equal to the discount rate
which would be applicable to a capital lease obligation with like term in
accordance with generally

                                       22
<PAGE>

accepted accounting principles) of the lessee for rental payments (other than
amounts required to be paid on account of property taxes, maintenance, repairs,
insurance, water rates and other items which do not constitute payments for
property rights) during the remaining portion of the initial term of the lease
included in such sale and leaseback transaction. (Section 101)

   "Consolidated Net Assets" is the aggregate amount of assets (less applicable
reserves and other properly deductible items) after deducting therefrom all
current liabilities (excluding certain renewable or extendible indebtedness) as
shown on the latest balance sheet of Monsanto and its consolidated subsidiaries
and computed in accordance with generally accepted accounting principles.
(Section 101)

   An "Operating Property" is any real property or equipment located within the
United States and used primarily for manufacturing by Monsanto or any of its
Subsidiaries that has a net book value (after deduction of accumulated
depreciation) in excess of 2.0% of Consolidated Net Assets, other than any such
property or equipment

     (1) which is financed by obligations issued by a state, commonwealth,
  territory or possession of the United States of America, or any political
  subdivision or governmental authority of any of the foregoing, or

     (2) which, in the opinion of Monsanto's Board of Directors, is not of
  material importance to the total business conducted by Monsanto and its
  Restricted Subsidiaries taken as a whole. (Section 101)

   A "Restricted Subsidiary" is any Subsidiary of Monsanto that owns any
Operating Property.

   A "Sale and Leaseback Transaction" is any arrangement with any bank,
insurance company or other lender or investor (other than Monsanto or another
Restricted Subsidiary) providing for the leasing by Monsanto or any Restricted
Subsidiary of any Operating Property (except a lease for a temporary period not
to exceed three years by the end of which it is intended that the use of such
Operating Property by the lessee will be discontinued), which was or is owned
or leased by Monsanto or a Restricted Subsidiary and which has been or is to be
sold or transferred, more than 120 days after the acquisition or the completion
of construction and commencement of full operation thereof, by Monsanto or such
Restricted Subsidiary to such lender or investor or to any Person to whom funds
have been or are to be advanced by such lender or investor on the security of
such Operating Property.

No Special Protection in the Event of a Highly Leveraged Transaction

   Like the holders of Monsanto's other public debt securities, Holders of Debt
Securities will have no special protection in the event of a highly leveraged
transaction.

Events of Default

   An Event of Default with respect to any series of debt securities is defined
in the indenture as:

  .  default in payment of principal of or premium, if any, on any debt
     security of that series at Maturity;

  .  default for 30 days in payment of interest on any debt security of that
     series;

  .  default for 30 days in the deposit of any sinking fund payment when due
     in respect of that series;

  .  failure by Monsanto in the performance of any other of the covenants or
     warranties in the indenture continued for 90 days after due notice by
     the trustee or by Holders of at least 25% in principal amount of the
     outstanding debt securities of that series;

  .  certain events of bankruptcy, insolvency or reorganization of Monsanto;
     and

  .  any other Event of Default provided with respect to debt securities of
     that series. (Section 501)

   The indenture provides that, if any Event of Default with respect to debt
securities of any series at the time outstanding occurs and is continuing,
either the trustee or the Holders of not less than 25% in principal amount of
the outstanding debt securities of that series may declare the principal amount
(or, if the debt

                                       23
<PAGE>

securities of that series are Original Issue Discount Securities or Indexed
Securities (as defined in the indenture), such portion of the principal amount
of such debt securities as may be specified in the terms thereof) of all debt
securities of that series to be due and payable immediately, but upon certain
conditions such declaration may be annulled and past defaults (except, unless
theretofore cured, a default in payment of principal of or premium, if any, or
interest, if any, on the debt securities of that series and certain other
specified defaults) may be waived by the Holders of a majority in principal
amount of the outstanding debt securities of that series on behalf of the
Holders of all debt securities of that series. (Sections 502 and 513)

   The indenture provides that the trustee will, within 90 days after the
occurrence of a default with respect to debt securities of any series at the
time outstanding, give to the Holders of the outstanding debt securities of
that series notice of such default known to it if uncured or not waived,
provided that, except in the case of default in the payment of principal of or
premium, if any, or interest on any debt security of that series, or in the
deposit of any sinking fund payment which is provided for, such notice shall
not be given until 30 days after the occurrence of a default with respect to
outstanding debt securities of such series. The term "default" with respect to
any series of outstanding debt securities for the purpose only of this
provision means the happening of any of the Events of Default specified in the
indenture and relating to such series of outstanding debt securities, excluding
any grace periods and irrespective of any notice requirements. (Section 602)

   The indenture contains a provision entitling the trustee, subject to the
duty of the trustee during default to act with the required standard of care,
to be indemnified by the Holders of any series of outstanding debt securities
before proceeding to exercise any right or power under the indenture at the
request of the Holders of such series of debt securities. (Section 603) The
indenture provides that the Holders of a majority in principal amount of
outstanding debt securities of any series may direct the time, method and place
of conducting any proceeding for any remedy available to the trustee, or
exercising any trust or other power conferred on the trustee, with respect to
the debt securities of such series provided that the trustee may decline to act
if such direction is contrary to law or the indenture or would expose it to
personal liability. (Section 512)

Defeasance of Debt Securities or Certain Covenants in Certain Circumstances

   Defeasance and Discharge. The terms of any series of debt securities will
provide that Monsanto will be discharged from any and all obligations in
respect of the debt securities of such series (except for certain obligations
to register the transfer or exchange of debt securities of such series, to
replace stolen, lost or mutilated debt securities of such series, to maintain
paying agencies and hold moneys for payment in trust) upon the deposit with the
trustee, in trust, of money and/or U.S. Government Obligations which, through
the payment of interest and principal thereof in accordance with their terms,
will provide money in an amount sufficient to pay and discharge the principal
of, and premium, if any, and interest on, and any mandatory sinking fund
payments applicable to, the debt securities of such series on the stated
maturity of such payments in accordance with the terms of the indenture and
such debt securities. Such discharge may only occur if, among other things,
Monsanto has delivered to the trustee an opinion of counsel to the effect that
Monsanto has received from, or there has been published by, the United States
Internal Revenue Service a ruling, or there has been a change in tax law, in
either case to the effect that such a discharge will not be deemed, or result
in, a taxable event with respect to Holders of the debt securities of such
series. (Article Thirteen)

   Defeasance of Certain Covenants. The terms of any series of debt securities
will provide Monsanto with the option to omit to comply with certain
restrictive covenants, including those described in Sections 801, 1007 and 1008
of the indenture. Monsanto, in order to exercise such option, will be required
to deposit with the trustee money and/or U.S. Government Obligations, which,
through the payment of interest and principal thereof in accordance with their
terms, will provide money in an amount sufficient to pay the principal of, and
premium, if any, and interest on, and mandatory sinking fund payments
applicable to the debt securities of such series on the stated maturity of such
payments in accordance with the terms of the indenture and such debt
securities. Monsanto will also be required to deliver to the trustee an opinion
of counsel to the effect that the deposit and related covenant defeasance will
not cause the Holders of the debt securities of such series to recognize
income, gain or loss for federal income tax purposes. In the event Monsanto
exercises this option and

                                       24
<PAGE>

the debt securities of such series are declared due and payable because of the
occurrence of any Event of Default, the amount of money and U.S. Government
Obligations, as the case may be, on deposit with the trustee will be sufficient
to pay amounts due on the debt securities of such series at the time of their
stated maturity but may not be sufficient to pay amounts due on the debt
securities of such series at the time of the acceleration resulting from such
Event of Default. However, Monsanto shall remain liable for such payments.

Modification of the Indenture and Waiver of Covenants

   The indenture contains provisions permitting Monsanto and the trustee, with
the consent of the Holders of not less than a majority in principal amount of
outstanding debt securities of each series affected thereby, to execute
supplemental indentures adding any provisions to or changing or eliminating any
of the provisions of the indenture or modifying the rights of the Holders of
outstanding debt securities of such series, except that no such supplemental
indenture may, without the consent of the Holder of each outstanding debt
security affected thereby:

  .  change the stated maturity, or reduce the principal amount, the premium,
     if any, thereon or the rate of payment of interest thereon, of any debt
     security of any series;

  .  reduce the aforesaid percentage of outstanding debt securities of any
     series, the consent of the Holders of which is required for any
     supplemental indenture or for waiver of compliance with certain
     provisions of the indenture or certain defaults thereunder; or

  .  effect certain other changes. (Section 902)

   The indenture also permits Monsanto to omit compliance with certain
covenants in the indenture with respect to debt securities of any series upon
waiver by the Holders of a majority in principal amount of outstanding debt
securities of such series.

Consolidation, Merger and Sale of Assets

   The indenture contains a provision permitting Monsanto, without the consent
of the Holders of any of the outstanding debt securities under the indenture,
to consolidate with or merge into any other corporation or transfer or lease
its assets substantially as an entirety to any person provided that:

  .  the successor is a corporation organized under the laws of any domestic
     jurisdiction;

  .  the successor corporation assumes Monsanto's obligations on the debt
     securities and under the indenture;

  .  after giving effect to the transaction, no Event of Default, and no
     event which, after notice or lapse of time, would become an Event of
     Default, shall have happened and be continuing; and

  .  certain other conditions are met. (Sections 801 and 802)

Concerning the Trustee

   The Bank of New York is the trustee under the indenture. The trustee is one
of a number of banks with which Monsanto and its subsidiaries maintain ordinary
banking and trust relationships. The trustee also serves as trustee or paying
agent under certain indentures pursuant to which Monsanto or its subsidiaries
have issued industrial revenue bonds, in an aggregate original principal amount
of approximately $137 million.

Governing Law

   The indenture for the debt securities and the debt securities, including the
registered debt, will be governed by New York law.

Global Securities

   The outstanding debt is, and the registered debt will be, issued in the form
of one or more global certificates, known as "global securities." The global
securities will be deposited on the date of the acceptance

                                       25
<PAGE>

for exchange of the outstanding debt and the issuance of the registered debt
with, or on behalf of, DTC and registered in the name of Cede & Co., as DTC's
nominee.

   Registered debt that is issued as described below under "Certificated
Securities" will be issued in the form of registered definitive certificates,
known as "certificated securities." Upon the transfer of certificated
securities, such certificated securities may, unless the global securities have
previously been exchanged for certificated securities, be exchanged for an
interest in the global securities representing the principal amount of
registered debt being transferred.

   Persons holding interests in the global securities may hold their interests
directly through DTC, or indirectly through organizations which are
participants in DTC. Transfers between participants will be effected in the
ordinary way in accordance with DTC rules and will be settled in immediately
available funds.

   Persons who are not participants may beneficially own interests in global
securities held by DTC only through participants or certain banks, brokers,
dealers, trust companies and other parties that clear through or maintain a
custodial relationship with a participant, either directly or indirectly, and
have indirect access to the DTC system. So long as Cede & Co., as DTC's
nominee, is the registered owner of any global security, it will be considered
the sole holder of such global security for all purposes. Except as provided
below, owners of beneficial interests in a global security will not be entitled
to have certificates registered in their names, will not receive or be entitled
to receive physical delivery of certificates in definitive form, and will not
be considered the holder thereof.

   None of Monsanto, the trustee nor any registrar or paying agent will have
any responsibility for the performance by DTC or its participants or indirect
participants of their respective obligations under the rules and procedures
governing their operations. DTC has advised Monsanto that it will take any
action permitted to be taken by a holder of registered debt only at the
direction of one or more participants whose accounts are credited with DTC
interests in a global security.

   DTC has advised Monsanto as follows: DTC is a limited purpose trust company
organized under the laws of the State of New York, a "banking organization"
within the meaning of the New York Banking Law, a member of the Federal Reserve
System, a "clearing corporation" within the meaning of the New York Uniform
Commercial Code and a "clearing agency" registered pursuant to the provisions
of Section 17A of the Exchange Act. DTC was created to hold securities for its
participants and to facilitate the clearance and settlement of securities
transactions, such as transfers and pledges, among participants in deposited
securities through electronic book-entry changes to accounts of its
participants, thereby eliminating the need for physical movement of securities
certificates. Participants include securities brokers and dealers, banks, trust
companies, clearing corporations and certain other organizations. Certain of
such participants or their representatives, together with other entities, own
DTC. The rules applicable to DTC and its participants are on file with the SEC.

   Purchases of registered debt under the DTC system must be made by or through
participants, which will receive a credit for the registered debt on DTC's
records. The ownership interest of each actual purchaser of registered debt,
referred to as a "beneficial owner," is in turn to be recorded on the
participants' and indirect participants' records. Beneficial owners will not
receive written confirmation from DTC of their purchase, but beneficial owners
are expected to receive written confirmations providing details of the
transaction, as well as periodic statements of their holdings, from the
participant or indirect participant through which the beneficial owner entered
into the transaction. Transfers of ownership interests in the registered debt
are to be accomplished by entries made on the books of participants acting on
behalf of beneficial owners. Beneficial owners will not receive certificates
representing their ownership interests in registered debt, except in the event
that use of the book-entry system for the registered debt is discontinued.

   The deposit of registered debt with DTC and their registration in the name
of Cede & Co. effect no change in beneficial ownership. DTC has no knowledge of
the actual beneficial owners of the registered debt; DTC's

                                       26
<PAGE>

records reflect only the identity of the participants to whose accounts such
registered debt is credited, which may or may not be the beneficial owners. The
participants will remain responsible for keeping account of their holdings on
behalf of their customers.

   The laws of some jurisdictions require that certain purchasers of securities
take physical delivery of securities in definitive form. Such laws may impair
the ability to transfer beneficial interests in the global security.

   Conveyance of notices and other communications by DTC to participants, by
participants to indirect participants and by participants and indirect
participants to beneficial owners will be governed by arrangements among them,
subject to any statutory or regulatory requirements that may be in effect from
time to time. Redemption notices shall be sent to Cede & Co. If less than all
of the registered debt is being redeemed, DTC's practice is to determine by lot
the interest of each participant in such registered debt to be redeemed.

   Principal and interest payments on the registered debt will be made to DTC
by wire transfer of immediately available funds. DTC's practice is to credit
participants' accounts on the payable date in accordance with their respective
holdings shown on DTC's records unless DTC has reason to believe that it will
not receive payment on the payable date. Payments by participants to beneficial
owners will be governed by standing instructions and customary practices, as is
the case with securities held for the accounts of customers in bearer form or
registered in "street name," and will be the responsibility of such participant
and not of DTC, or Monsanto, subject to any statutory or regulatory
requirements as may be in effect from time to time. Payment of principal and
interest to DTC is Monsanto's responsibility, disbursement of such payments to
participants shall be DTC's responsibility, and disbursement of such payments
to the beneficial owners shall be the responsibility of participants and
indirect participants. Neither Monsanto nor the trustee will have any
responsibility or liability for any aspect of the records relating to or
payments made on account of beneficial ownership interests in the global
securities or for maintaining, supervising or reviewing any records relating to
such beneficial ownership interests.

   DTC may discontinue providing its services as securities depositary with
respect to the registered debt at any time by giving reasonable notice to
Monsanto. In the event that DTC notifies Monsanto that it is unwilling or
unable to continue as depositary for any global security or if at any time DTC
ceases to be a clearing agency registered as such under the Exchange Act when
DTC is required to be so registered to act as such depositary and no successor
depositary shall have been appointed within 90 days of such notification or of
Monsanto becoming aware of DTC's ceasing to be so registered, as the case may
be, certificates for the registered debt will be printed and delivered in
exchange for interests in such global security. Any global security that is
exchangeable pursuant to the preceding sentence shall be exchangeable for
registered debt registered in such names as DTC shall direct. It is expected
that such instructions will be based upon directions received by DTC from its
participants with respect to ownership of beneficial interests in such global
security.

   Monsanto may decide to discontinue use of the system of book-entry transfers
through DTC or a successor securities depositary. In that event, certificates
representing the registered debt will be printed and delivered.

   The information in this section concerning DTC and DTC's book-entry system
has been obtained from sources that Monsanto believes to be reliable, but
Monsanto does not take responsibility for the accuracy thereof.

Certificated Securities

   Subject to certain conditions, any person having a beneficial interest in
the global securities may, upon request to the trustee, exchange such
beneficial interest for registered debt in the form of certificated securities.
Upon any such issuance, the trustee is required to register such certificated
securities in the name of, and cause the same to be delivered to, such person
or persons, or their nominee, if applicable. In addition, if (1) Monsanto
notifies the trustee in writing that DTC is no longer willing or able to act as
a depositary and Monsanto is

                                       27
<PAGE>

unable to locate a qualified successor within 90 days or (2) Monsanto, at its
option, notifies the trustee in writing that it elects to cause the issuance of
registered debt in the form of certificated securities under the indenture,
then, upon surrender by Cede & Co., or its nominee, of global securities,
registered debt in such form will be issued to each person that Cede & Co., or
its nominee, and DTC identify as being the beneficial owner of the related
registered debt.

   Neither Monsanto nor the trustee will be liable for any delay by Cede & Co.,
or its nominee, or DTC in identifying the beneficial owners of registered debt
and Monsanto and the trustee may conclusively rely on, and will be protected in
relying on, instructions from Cede & Co., or its nominee, or DTC for all
purposes.

Year 2000

   The following information has been provided by DTC:

   DTC management is aware that some computer applications, systems, and the
like for processing data that are dependent upon calendar dates, including
dates before, on, and after January 1, 2000, may encounter "Year 2000
problems." DTC has informed its participants and other members of the financial
community that it has developed and is implementing a program so that its
computer systems, as the same relate to the timely payment of distributions
(including principal and income payments) to securityholders, book-entry
deliveries, and settlement of trades within DTC, continue to function
appropriately. This program includes a technical assessment and a remediation
plan, each of which is complete. Additionally, DTC's plan includes a testing
phase, which is expected to be completed within appropriate time frames.

   However, DTC's ability to perform properly its services is also dependent
upon other parties, including but not limited to issuers and their agents, as
well as third-party vendors from whom DTC licenses software and hardware, and
third-party vendors on whom DTC relies for information or the provision of
services, including telecommunication and electrical utility service providers,
among others. DTC has informed the financial community that it is contacting,
and will continue to contact, third-party vendors from whom DTC acquires
services to: (1) impress upon them the importance of such services being Year
2000 compliant; and (2) determine the extent of their efforts for Year 2000
remediation and, as appropriate, testing of their services. In addition, DTC is
in the process of developing such contingency plans as it deems appropriate.

   According to DTC, the foregoing information with respect to DTC has been
provided to the Industry for informational purposes only and is not intended to
serve as a representation, warranty, or contract modification of any kind.

Registration Rights; Liquidated Damages

   Monsanto entered into a registration rights agreement with the initial
purchasers, dated December 9, 1998, for the benefit of the holders of the
outstanding debt wherein Monsanto agreed, for the benefit of the holders of the
outstanding debt, (1) to file with the SEC a registration statement, of which
this prospectus forms a part, with respect to the registered debt and (2) to
use its reasonable efforts to cause such exchange offer registration statement
to be declared effective under the Securities Act on or before June 7, 1999. If

  .  because of any change in law or in currently prevailing interpretations
     of the SEC's staff, Monsanto is not permitted to effect the exchange
     offers, or

  .  in the case of any holder that participates in the exchange offers, such
     holder does not receive applicable registered debt on the date of the
     exchange that may be sold without restriction under state and Federal
     securities laws (other than due solely to the status of such holder as
     an affiliate of Monsanto within the meaning of the Securities Act or as
     a broker-dealer),

                                       28
<PAGE>

then in each case, Monsanto will (1) promptly deliver to the holders written
notice thereof and (2) at Monsanto's sole expense (a) as promptly as
practicable, file a shelf registration statement covering resales of the
applicable outstanding debt, (b) use its reasonable efforts to cause the shelf
registration statement to be declared effective under the Securities Act and
(c) use its reasonable efforts to keep effective the shelf registration
statement until the earlier of December 9, 2000, unless Rule 144(k) is amended
to provide a shorter restrictive period, or such time as all of the outstanding
debt or registered debt, as applicable, has been sold thereunder.

   Monsanto will, in the event that a shelf registration statement is filed,
provide to each applicable holder copies of the prospectus that is a part of
the shelf registration statement, notify each such holder when the shelf
registration statement for the outstanding debt or registered debt, as
applicable, has become effective and take certain other actions as are required
to permit unrestricted resales of the outstanding debt. A holder that sells
outstanding debt or registered debt, as applicable, pursuant to the shelf
registration statement will be required to be named as a selling security
holder in the related prospectus and to deliver a prospectus to purchasers,
will be subject to certain of the civil liability provisions under the
Securities Act in connection with such sales and will be bound by the
provisions of the registration rights agreement that are applicable to such a
holder, including certain indemnification rights and obligations.

   The registration rights agreement requires Monsanto to pay liquidated
damages to holders of the outstanding debt from June 8, 1999 until it
consummates the exchange offer. In addition, if Monsanto fails to comply with
certain other provisions of the registration rights agreement, in each case as
described below, then liquidated damages shall become payable in respect of the
outstanding debt as follows:

   In the event that the exchange offer is not consummated or the applicable
shelf registration statement is not declared effective on or prior to July 22,
1999, liquidated damages shall accrue on the applicable outstanding debt from
and including July 23, 1999 at a rate equal to 0.25% per year. The aggregate
amount of liquidated damages payable pursuant to the above provision will in no
event exceed 0.25% per year. Upon the consummation of the applicable exchange
offer or the effectiveness of the applicable shelf registration statement, as
the case may be, after July 22, 1999, the liquidated damages will cease to
accrue.

   If a shelf registration statement is declared effective pursuant to the
foregoing paragraphs and Monsanto fails to keep it continuously (1) effective
or (2) useable for resales for the period required by the registration rights
agreement due to certain circumstances relating to pending corporate
developments, public filings with the SEC and similar events, or because the
prospectus contains an untrue statement of a material fact or omits to state a
material fact required to be stated therein or necessary in order to make the
statements therein not misleading, and such failure continues for more than 60
days, whether or not consecutive, in any 12-month period (the 61st day being
referred to as the "Default Day"), then liquidated damages shall accrue at a
rate equal to 0.25% per year from the Default Day until the earlier of

  .  the date that the shelf registration statement is again deemed effective
     or is useable,

  .   December 9, 2001, unless Rule 144(k) is amended to provide a shorter
     restrictive period, or

  .  the date as of which all of the applicable outstanding debt is sold
     pursuant to the shelf registration statement.

   Any amounts of liquidated damages due pursuant to the foregoing paragraphs
will be payable in cash on June 1 and December 1 of each year to the holders of
record on the preceding May 15 and November 15, respectively.

   The registration rights agreement is governed by, and construed in
accordance with, the laws of the State of New York. This summary of certain
provisions of the registration rights agreement is not complete and is subject
to, and is qualified in its entirety by reference to, all the provisions of the
registration rights agreement. In addition, the information set forth above
concerning certain interpretations of and positions taken by the SEC's staff is
not intended to constitute legal advice, and holders of outstanding debt should
consult their own legal advisors with respect to such matters.

                                       29
<PAGE>

                     UNITED STATES FEDERAL TAX CONSEQUENCES

   The following is a discussion of the material United States federal tax
consequences associated with the payment of liquidated damages, the exchange of
the outstanding debt for the registered debt pursuant to the exchange offer and
the disposition of the registered debt. This discussion applies only to a
beneficial owner of registered debt who acquired outstanding debt at the
initial offering from Salomon Smith Barney Inc. and Goldman, Sachs & Co. for
the original offering price thereof and who acquires the registered debt
pursuant to the exchange offer. This discussion is based upon the United States
federal tax law now in effect, which is subject to change, possibly
retroactively. This discussion does not consider any specific facts or
circumstances that may apply to a particular holder. Prospective investors are
urged to consult their tax advisors regarding the United States federal tax
consequences of the receipt of liquidated damages and the acquiring, holding,
and disposing of the registered debt, as well as any tax consequences that may
arise under the laws of any foreign, state, local, or other taxing
jurisdiction.

   For purposes of this discussion, a "U.S. holder" means a holder of
registered debt that is either a citizen or resident of the United States, a
corporation, partnership, or other entity created or organized in the United
States or under the laws of the United States or of any political subdivision
thereof, an estate whose income is includible in gross income for United States
federal income tax purposes regardless of its source, or a trust whose
administration is subject to the primary supervision of a United States court
and for which one or more United States persons have the authority to control
all substantial decisions of the trust. A "non-U.S. holder" is a holder of
registered debt other than a U.S. holder.

Exchange Offer

   The exchange of outstanding debt for registered debt pursuant to the
exchange offer will not constitute a "significant modification" of the
outstanding debt for United States federal income tax purposes and,
accordingly, the registered debt received will be treated as a continuation of
the outstanding debt in the hands of such holder. As a result, there will be no
United States federal income tax consequences to a U.S. holder who exchanges
outstanding debt for registered debt pursuant to the exchange offer, and any
such holder will have the same adjusted tax basis and holding period in the
registered debt for United States federal income tax purposes as it had in the
outstanding debt immediately before the exchange.

Stated Interest/Original Issue Discount

   Monsanto took the position with respect to the outstanding debt (which was
generally binding upon holders) that, at the time of the original issuance of
the outstanding debt, the possibility of the payment of liquidated damages on
such debt was "remote" within the meaning of applicable Treasury regulations.
Based in part upon this determination of the remoteness of liquidated damages,
Monsanto took the position that the outstanding debt was not issued with
original issue discount. However, despite this determination at the time of
original issuance, liquidated damages have accrued on the outstanding debt
since June 7, 1999.

   Monsanto intends to take the position (which will generally be binding upon
holders) that the liquidated damages constitute a separate payment of damages
for failure to comply with certain covenants within the outstanding debt, and
are to be taken into account by holders of the outstanding debt in accordance
with their method of accounting for tax purposes. Consequently, Monsanto
intends to take the position that the liquidated damages are not payments to be
taken into account in determining whether the outstanding debt (or the
registered debt as a continuation of the outstanding debt) has original issue
discount. Nevertheless, the Internal Revenue Service may not agree with this
position and may successfully assert that the liquidated damages are in fact
payments on both the outstanding debt and consequently the registered debt as a
continuation of the outstanding debt.

   If the Internal Revenue Service were successful in this assertion, then
solely for purposes of determining whether the outstanding debt and registered
debt have original issue discount, the accrual of liquidated damages

                                       30
<PAGE>


probably caused the deemed retirement and reissuance of the outstanding debt on
June 7, 1999 for an amount of money equal to its then adjusted issue price. The
adjusted issue price of the reissued outstanding debt on June 7, 1999 would
have been equal to the initial price at which the outstanding debt was issued,
minus any payments of principal made to such date. This deemed reissuance
requires that the outstanding debt be examined again on June 7, 1999 to
determine whether it was reissued with original issue discount. As reissued
debt for purposes of determining original issue discount, the outstanding debt
would include the current accrual and payment of liquidated damages as part of
its terms.

   In such case, under the general rules, the reissued outstanding debt
probably should be treated as a "contingent payment debt instrument." As a
contingent payment debt instrument, the stated interest on the reissued
outstanding debt, as well as expected liquidated damages (as contingent
payments), generally would be taken into account in determining the comparable
yield of a debt instrument with a single fixed rate of interest and similar
terms and conditions to that of the reissued outstanding debt. Using this
comparable yield and a projected payment schedule for all payments required to
be paid on the reissued outstanding debt, as well as liquidated damages (as
contingent payments), a new assumed accrual of interest (as original issue
discount) would be determined for each accrual period for the reissued
outstanding debt. Interest on the reissued outstanding debt would generally be
accrued by holders on the basis of this new accrual regardless of their method
of tax accounting, subject to adjustment if the actual liquidated damages paid
are different from that assumed in the projected payment schedule.

   Further in such case, even though no liquidated damages would be paid for
periods after the registered debt is issued, the registered debt would be
treated as a continuation of the reissued outstanding debt for the purpose of
original issue discount. Consequently, the registered debt would be treated as
having original issue discount, and holders of registered debt would be
required to accrue interest as original issue discount based upon the projected
comparable yield described above for the reissued outstanding debt regardless
of their method of tax accounting.

   Finally, if the actual liquidated damages paid for a taxable year are more
than that assumed in the projected payment schedule, such difference would be a
"net positive adjustment"; if the actual liquidated damages paid for a taxable
year are less than assumed in such case for the projected payment schedule,
such difference would be a "net negative adjustment." A net positive adjustment
for a taxable year would be required to be included in gross income by a holder
of the reissued outstanding/registered debt as additional interest income. A
net negative adjustment for a taxable year would first reduce accrued interest
on the reissued outstanding/registered debt for the taxable year and would be
recognized by holders as an ordinary loss to the extent the aggregate net
negative adjustments exceed the total interest included by holders on the
reissued outstanding/registered debt. (Any excess net negative adjustment is
carried forward on the reissued outstanding/registered debt.)

   The potential for the deemed reissuance of the outstanding debt, and
resulting treatment of the outstanding debt and registered debt as having
original issue discount present particularly complex tax issues; all holders
should consult their tax advisors concerning these issues.

Disposition

   A holder of registered debt will recognize gain or loss upon the sale,
exchange, redemption or other taxable disposition of the registered debt
measured by the difference between (1) the amount of cash and fair market value
of property received (that is not attributable to accrued interest which has
not yet been recognized as gross income) and (2) the holder's adjusted tax
basis in the registered debt. Any such gain or loss generally will be long-term
capital gain or loss, provided the registered debt was a capital asset in the
hands of the holder and had been held for more than one year.

                                       31
<PAGE>


   However, if the Internal Revenue Service were to successfully assert that
the outstanding debt was reissued on June 7, 1999 as a contingent payment debt
instrument (as described above under the heading "United States Federal Tax
Consequences--Stated Interest/Original Issue Discount"), any gain recognized by
a holder of either the reissued outstanding debt or the registered debt prior
to the final payment of liquidated damages should be recognized as interest
income. Any loss recognized by a holder on the outstanding debt or the
registered debt prior to the final payment of liquidated damages should be
ordinary to the extent that the total interest inclusions on the reissued
outstanding/registered debt exceed the total net negative adjustments taken
into account by the holder.

Non-U.S. Holders

   Under present United States federal income and estate tax law, assuming
certain certification requirements are satisfied (which include identification
of the beneficial owner of the instrument), and subject to the discussion of
backup withholding below:

     (1) payments of interest on the registered debt to any non-U.S. holder
  will not be subject to United States federal income or withholding tax,
  provided that (a) the holder does not actually or constructively own 10% or
  more of the total combined voting power of all classes of stock of Monsanto
  entitled to vote,
  (b) the holder is not (i) a bank receiving interest pursuant to a loan
  agreement entered into in the ordinary course of its trade or business or
  (ii) a controlled foreign corporation that is related to Monsanto through
  stock ownership, and (c) such interest payments are not effectively
  connected with the conduct of a United States trade or business of the
  holder;

     (2) a holder of registered debt who is a non-U.S. holder will not be
  subject to the United States federal income tax on gain realized on the
  sale, exchange, or other disposition of registered debt, unless (a) such
  holder is an individual who is present in the United States for 183 days or
  more during the taxable year and certain other requirements are met, or (b)
  the gain is effectively connected with the conduct of a United States trade
  or business of the holder; and

     (3) if interest on the registered debt is exempt from withholding of
  United States federal income tax under the rules described above (without
  regard to the certification requirement), the registered debt will not be
  included in the estate of a deceased non-U.S. holder for United States
  Federal estate tax purposes.

   The certification referred to above may be made on an Internal Revenue
Service Form W-8 or a substantially similar substitute form.

Information Reporting and Backup Withholding

   Monsanto will, where required, report to the holders of registered debt and
the Internal Revenue Service the amount of any interest paid on the registered
debt in each calendar year and the amounts of federal tax withheld, if any,
with respect to such payments. A noncorporate U.S. holder may be subject to
information reporting and to backup withholding at a rate of 31% with respect
to payments of principal and interest made on registered debt, or on proceeds
of the disposition of registered debt before maturity, unless such U.S. holder
provides a correct taxpayer identification number or proof of an applicable
exemption, and otherwise complies with applicable requirements of the
information reporting and backup withholding rules.

   Under temporary United States Treasury regulations, United States
information reporting requirements and backup withholding tax will generally
not apply to interest paid on the registered debt to a non-U.S. holder at an
address outside the United States. Payments by a United States office of a
broker of the proceeds of a sale of the registered debt are subject to both
backup withholding at a rate of 31% and information reporting unless the holder
certifies its non-U.S. holder status under penalties of perjury and provides
its name and address or otherwise establishes an exemption. Information
reporting requirements (but not backup withholding) will also apply to payments
of the proceeds of sales of the registered debt by foreign offices of United
States brokers, or foreign brokers with certain types of relationships to the
United States, unless the broker has documentary evidence in its records that
the holder is a non-U.S. holder and certain other conditions are met, or the
holder otherwise establishes an exemption.

                                       32
<PAGE>

   Backup withholding is not an additional tax. Any amount withheld under the
backup withholding rules will be refunded or credited against the non-U.S.
holder's United States Federal income tax liability, provided that the required
information is furnished to the Internal Revenue Service.

New Treasury Regulations Applicable to Non-U.S. Holders

   The United States Treasury Department has issued final Treasury regulations
governing information reporting and the certification procedures regarding
withholding and backup withholding on certain amounts paid to non-U.S. holders
after December 31, 2000. The new Treasury regulations modify and, in general,
unify the way in which non-U.S. holders may establish eligibility for United
States federal withholding tax exemptions, including that under a tax treaty,
and an exemption from backup withholding.

   For example, the new Treasury regulations will require new forms, which non-
U.S. holders will generally have to provide earlier than you would have had to
provide replacements for expiring existing forms. The new Treasury regulations
also clarify the standards upon which withholding agents of non-U.S. holders
may rely, add requirements in order for non-U.S. holders to claim reduced
federal tax withholding under a tax treaty, and provide different procedures in
order for foreign intermediaries and flow-through entities (such as foreign
partnerships) to claim the benefit of applicable exemptions if they receive
payments on behalf of non-U.S. holders.

   The new Treasury regulations are particularly complex. Non-U.S. holders
should consult their tax advisors concerning the effect, if any, of such new
Treasury regulations on their investment in the registered debt.

                                       33
<PAGE>

                              PLAN OF DISTRIBUTION

   Based on existing interpretations of the Securities Act by the SEC's staff
set forth in several no-action letters to third parties, and subject to the
immediately following sentence, Monsanto believes that the registered debt
issued in the exchange offer may be offered for resale, resold and otherwise
transferred by the holders thereof, other than holders who are broker-dealers,
without further compliance with the registration and prospectus delivery
provisions of the Securities Act. However, any purchaser of outstanding debt
who is an affiliate of Monsanto or who intends to participate in the exchange
offer for the purpose of distributing the registered debt, or any broker-dealer
who purchased the outstanding debt from Monsanto to resell pursuant to Rule
144A or any other available exemption under the Securities Act, (1) will not be
able to rely on the interpretations of the SEC's staff set forth in the above-
mentioned no-action letters, (2) will not be entitled to tender its outstanding
debt in the exchange offer and (3) must comply with the registration and
prospectus delivery requirements of the Securities Act in connection with any
sale or transfer of the outstanding debt, unless such sale or transfer is made
pursuant to an exemption from such requirements. Monsanto does not intend to
seek its own no-action letter, and there can be no assurance that the SEC's
staff would make a similar determination with respect to the registered debt as
it has in such no-action letters to third parties.

   Each holder of the outstanding debt, other than certain specified holders,
who wishes to exchange the outstanding debt for registered debt in the exchange
offer is required to represent that:

  .   it is not an affiliate of Monsanto;

  .  the registered debt to be received by it was acquired in the ordinary
     course of its business;

  .  at the time of the exchange offer, it has no arrangement with any person
     to participate in the "distribution," within the meaning of the
     Securities Act, of the registered debt; and

  .  if such holder is not a broker-dealer, it is not engaged in, and does
     not intend to engage in, a "distribution," within the meaning of the
     Securities Act, of the registered debt.

  In addition, in connection with any resales of registered debt, any broker-
dealer who acquired the outstanding debt for its own account as a result of
market-making or other trading activities, referred to in this section as a
"participating broker-dealer," must deliver a prospectus meeting the
requirements of the Securities Act. Based on the position the SEC has taken to
date, Monsanto believes that participating broker-dealers may fulfill their
prospectus delivery requirements with respect to the exchange debt, other than
a resale of an unsold allotment from the original sale of the outstanding debt,
with this prospectus. Under the registration rights agreement, Monsanto is
required to allow participating broker-dealers and other persons, if any,
subject to similar prospectus delivery requirements to use the prospectus in
connection with the resale of such registered debt for a period of 180 days
from the expiration date of the exchange offer.

   Each broker-dealer that receives registered debt for its own account
pursuant to the exchange offer must acknowledge that it will deliver a
prospectus in connection with any resale of such registered debt. This
prospectus, as it may be amended or supplemented from time to time, may be used
by a broker-dealer in connection with resales of registered debt received in
exchange for outstanding debt where such outstanding debt was acquired by such
broker-dealer as a result of market-making activities or other trading
activities. Each broker-dealer agrees that it shall suspend use of this
prospectus upon notice from Monsanto of the occurrence of certain events until
Monsanto has amended or supplemented the prospectus so that it does not contain
any untrue statements or omissions of material facts.

   Monsanto will not receive any proceeds from any sale of registered debt by
broker-dealers. Registered debt received by broker-dealers for their own
account pursuant to the exchange offfer may be sold from time to time in one or
more transactions in the over-the-counter market, in negotiated transactions,
through the writing of options on the registered debt or a combination of such
methods of resale, at market prices prevailing at the time of resale, at prices
related to such prevailing market prices or at negotiated prices. Any such
resale may be made directly to purchasers or to or through brokers or dealers
who may receive compensation in the form of commissions or concessions from any
such broker-dealer and/or the purchasers of any such registered debt.

                                       34
<PAGE>

Any broker-dealer that resells registered debt that was received by it for its
own account pursuant to the exchange offer and any broker or dealer that
participates in a distribution of such registered debt may be deemed to be an
"underwriter" within the meaning of the Securities Act and any profit of any
such resale of registered debt and any commissions or concessions received by
any such persons may be deemed to be underwriting compensation under the
Securities Act. Each letter of transmittal states that by acknowledging that it
will deliver and by delivering a prospectus, a broker-dealer will not be deemed
to admit that it is an "underwriter" within the meaning of the Securities Act.

   For a period of 180 days after the expiration date, Monsanto will promptly
send additional copies of this prospectus and any amendment or supplement to
this prospectus to any broker-dealer that requests such documents in the
applicable letter of transmittal. Monsanto has agreed to pay all expenses
incident to the exchange offer other than commissions or concessions of any
brokers or dealers and will indemnify the holders of the outstanding debt,
including any broker-dealers, against certain liabilities, including
liabilities under the Securities Act.

                      WHERE YOU CAN FIND MORE INFORMATION

   Monsanto files annual, quarterly and special reports, proxy statements and
other information with the SEC. Monsanto's filings with the SEC are available
to the public over the Internet at the SEC's web site at http://www.sec.gov.
You may also read and copy any document Monsanto files with the SEC at the
SEC's public reference rooms in Washington, D.C., New York, New York and
Chicago, Illinois. You may call the SEC at 1-800-SEC-0330 for further
information on the public reference rooms.

   Monsanto's reports, proxy statements and other information may also be
inspected at the offices of the New York Stock Exchange, the exchange on which
certain of Monsanto's securities are listed.

   No person is authorized to give any information or to make any
representations with respect to the matters described in this prospectus other
than those contained herein or in the documents incorporated by reference
herein. Any information or representation with respect to such matters not
contained herein or therein must not be relied upon as having been authorized
by Monsanto. The delivery of this prospectus shall not, under any
circumstances, create any implication that there has been no change in the
affairs of Monsanto since the date hereof or that the information in this
prospectus or in the documents incorporated by reference herein is correct as
of any time subsequent to the date hereof or thereof.

                INCORPORATION OF CERTAIN DOCUMENTS BY REFERENCE

   The SEC allows Monsanto to "incorporate by reference" certain information
that Monsanto files with the SEC, which means that Monsanto can disclose
important information to you by referring you to information in those
documents. The information incorporated by reference is an important part of
this prospectus. The following documents and other materials, which Monsanto
has filed with the SEC, are incorporated herein and specifically made a part
hereof by this reference:

     (1) Monsanto's Annual Reports on Form 10-K and Form 10-K/A filed with
  the SEC on March 25, 1999 and January 21, 2000, respectively;

     (2) Monsanto's Quarterly Reports on Form 10-Q filed with the SEC on May
  17, August 16 and November 15, 1999 and Quarterly Reports on Form 10-Q/A
  each filed with the SEC on January 21, 2000; and

     (3) Monsanto's Current Reports on Form 8-K/A filed with the SEC on
  February 8, 1999 and January 25, 2000 and Current Reports on Form 8-K filed
  with the SEC on March 1, May 4, December 21, December 22, December 29, and
  December 30, 1999 and January 11 and January 25, 2000.

   In addition, (1) all documents filed with the SEC pursuant to Section 13(a),
13(c), 14 and 15(d) of the Exchange Act by Monsanto subsequent to the date of
this prospectus and (2) all documents filed with the SEC

                                       35
<PAGE>

pursuant to the Exchange Act after the date of the initial registration
statement of which this prospectus is a part and prior to the effectiveness of
such registration statement shall be deemed to be incorporated by reference
into this prospectus and to be a part hereof from the date of filing of such
documents with the SEC. Any statement contained herein or in a document
incorporated or deemed to be incorporated by reference herein shall be deemed
to be modified or superseded for purposes of this prospectus to the extent that
a statement contained herein or in any other subsequently filed document which
also is or is deemed to be incorporated by reference herein modifies or
supersedes such statement. Any such statement so modified or superseded shall
not be deemed, except as so modified or superseded, to constitute a part of
this prospectus.

   Statements contained in this prospectus or in any document incorporated by
reference in this prospectus as to the contents of any contract or other
document referred to herein or therein are not necessarily complete, and in
each instance reference is made to the copy of such contract or other document
filed as an exhibit to the documents incorporated by reference, each such
statement being qualified in all respects by such reference.

   The forward-looking information included in the press releases dated
December 19, 1999 and December 22, 1999 and the analyst presentations dated
December 19, 1999 and December 22, 1999 which are included in the Monsanto
Current Reports on Form 8-K filed on December 21, 1999 and December 22, 1999
are based on internal forecasts prepared by the managements of Monsanto and
Pharmacia & Upjohn, adjusted to give effect to the merger, and were initially
prepared for the Boards of Directors of the respective companies and
subsequently disclosed to analysts and included in press releases. The forward-
looking information is not intended to comply with the presentation and
disclosure guidelines for prospective financial information of the SEC or the
American Institute of Certified Public Accountants.

   The forward-looking information included in the Monsanto Current Reports on
Form 8-K filed on December 21, 1999 and December 22, 1999, which are
incorporated by reference in this prospectus, has been prepared by, and is the
responsibility of the managements of Monsanto and Pharmacia & Upjohn. Deloitte
& Touche LLP and PricewaterhouseCoopers LLP have not examined, compiled or
performed any procedures with respect to the forward-looking information and,
accordingly, Deloitte & Touche LLP and PricewaterhouseCoopers LLP do not
express an opinion or any other form of assurance with respect thereto and
disclaim any association with, the forward-looking information. The Deloitte &
Touche LLP and PricewaterhouseCoopers LLP reports incorporated by reference in
this prospectus relate to the historical financial information of Monsanto and
Pharmacia & Upjohn, respectively. The reports do not extend to the forward-
looking information and should not be read to do so.

   This prospectus incorporates documents by reference that are not presented
herein or delivered herewith. Copies of such documents, other than exhibits to
such documents that are not specifically incorporated by reference herein, are
available without charge to any person to whom this prospectus is delivered,
upon written or oral request to: Monsanto Company, 800 North Lindbergh
Boulevard, St. Louis, Missouri 63167, Attention: Corporate Secretary,
telephone: (314) 694-1000.

                                       36
<PAGE>

                        VALIDITY OF THE REGISTERED DEBT

   The validity of the registered debt offered hereby will be passed upon for
Monsanto by Winston & Strawn, Chicago, Illinois.

                                    EXPERTS

   The consolidated financial statements incorporated by reference in
Monsanto's amended Annual Report on Form 10-K/A for the year ended December 31,
1998 have been audited by Deloitte & Touche LLP, independent auditors, as
stated in their report, which is incorporated herein by reference, and have
been so incorporated herein in reliance upon such report of such firm given
upon their authority as experts in accounting and auditing.

   The consolidated financial statements of DEKALB Genetics Corporation and
subsidiaries incorporated by reference in this prospectus and elsewhere in the
registration statement have been audited by Arthur Andersen LLP, independent
public accountants, as indicated in their report with respect thereto, and are
included herein in reliance upon the authority of said firm as experts in
giving said report.

   The financial statements of Pharmacia & Upjohn incorporated in this
Registration Statement by reference to Monsanto's Current Report on Form 8-K
filed on January 25, 2000, have been so incorporated in reliance on the report
of PricewaterhouseCoopers LLP, independent accountants, given on the authority
of said firm as experts in auditing and accounting.

                                       37
<PAGE>

- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------

                                Monsanto Company

                               Exchange Offer for

                       $500,000,000 5.375% Notes due 2001
                       $600,000,000 5.750% Notes due 2005
                       $200,000,000 5.875% Notes due 2008
                    $500,000,000 6.500% Debentures due 2018
                    $700,000,000 6.600% Debentures due 2028

                                [Monsanto logo]

                               ----------------

                                   PROSPECTUS

                               ----------------

                                      , 2000

- --------------------------------------------------------------------------------
- --------------------------------------------------------------------------------
<PAGE>

                                    PART II

                     INFORMATION NOT REQUIRED IN PROSPECTUS

ITEM 20. INDEMNIFICATION OF DIRECTORS AND OFFICERS

   Section 145 of the General Corporation Law of the State of Delaware permits
indemnification of directors, officers, employees and agents of corporations
under certain conditions and subject to certain limitations. Section 59 of the
Company's By-Laws provides for indemnification of any director, officer,
employee or agent of the Company, or any person serving in the same capacity in
any other enterprise at the request of the Company, under certain
circumstances. Article IX of the Company's Restated Certificate of
Incorporation eliminates the liability of directors of the Company under
certain circumstances for breaches of fiduciary duty to the Company and its
shareholders.

   The Company maintains directors' and officers' liability insurance coverage.

ITEM 21. EXHIBITS AND FINANCIAL STATEMENT SCHEDULES

   The following documents are filed herewith or incorporated herein by
reference.

<TABLE>
<CAPTION>
 Exhibit
 Number                          Description of Exhibits
 -------                         -----------------------
 <C>     <S>
  3.1*   Restated Certificate of Incorporation of the Company as of October 28,
         1997 (incorporated herein by reference to Exhibit 3(i) of the
         Company's Form 10-Q for the quarter ended September 30, 1997 (File No.
         1-2516))

  3.2    By-Laws of the Company, as amended, effective December 10, 1999
         (incorporated herein by reference to Exhibit 3.2 of the Company's Form
         10-K/A for the year ended December 31, 1998 (File No. 1-2516))

  4.1*   Indenture dated as of December 1, 1998 between the Company and The
         Bank of New York as Trustee, providing for Issuance of Debt Securities
         in Series (incorporated herein by reference to Exhibit 4.7 of the
         Company's Current Report on Form 8-K as filed with the Commission on
         December 14, 1998 (File No. 1-2516))

  4.2*   Form of Officer's Certificate, establishing the terms of the Company's
         5.375% Notes due 2001, 5.750% Notes due 2005, 5.875% Notes due 2008,
         6.500% Debentures due 2018 and 6.600% Debentures due 2028

  4.3*   Registration Rights Agreement, dated as of December 9, 1998, by and
         among the Company, Salomon Smith Barney Inc. and Goldman, Sachs & Co.

  5.1*   Opinion of Winston & Strawn

 12.1    Computation of Ratios of Earnings to Fixed Charges (for last five
         fiscal years) (incorporated herein by reference to Exhibit 99 of the
         Company's Form 10-Q/A for the quarter ended September 30, 1999 (File
         No. 1-2516))

 23.1    Consent of Deloitte & Touche LLP

 23.2*   Consent of Winston & Strawn (included in Exhibit 5.1)

 23.3    Consent of Arthur Andersen LLP

 23.4    Consent of PricewaterhouseCoopers LLP

 24.1*   Powers of Attorney

 24.2*   Powers of Attorney

 25.1*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.375% Notes due 2001

 25.2*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.750% Notes due 2005
</TABLE>

                                      II-1
<PAGE>

<TABLE>
<CAPTION>
 Exhibit
 Number                        Description of Exhibits
 -------                       -----------------------
 <C>     <S>
 25.3*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.875% Notes due 2008

 25.4*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 6.500% Debentures due 2018

 25.5*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 6.600% Debentures due 2028

 99.1*   Form of Letter of Transmittal

 99.2*   Form of Notice of Guaranteed Delivery

 99.3*   Form of Tender Instructions
</TABLE>
- --------
  *Previously filed.

ITEM 22. UNDERTAKINGS

   (1) The Company hereby undertakes that, for purposes of determining any
liability under the Securities Act of 1933 (the "Securities Act"), each filing
of the Company's annual report pursuant to Section 13(a) or Section 15(d) of
the Securities Exchange Act of 1934 (the "Exchange Act")(and, where applicable,
each filing of an employee benefit plan's annual report pursuant to Section
15(d) of the Exchange Act) that is incorporated by reference in the
Registration Statement shall be deemed to be a new Registration Statement
relating to the securities offered therein, and the offering of such securities
at that time shall be deemed to be the initial bona fide offering thereof.

   (2) The Company hereby undertakes to respond to requests for information
that is incorporated by reference into the Prospectus pursuant to Items 4,
10(b), 11, or 13 of Form S-4, within one business day of receipt of such
request, and to send the incorporated documents by first class mail or other
equally prompt means. This includes information contained in documents filed
subsequent to the effective date of the Registration Statement through the date
of responding to the request.

   (3) The Company hereby undertakes to supply by means of a post-effective
amendment all information concerning a transaction, and the company being
acquired involved therein, that was not the subject of and included in the
Registration Statement when it became effective.

   (4) Insofar as indemnification for liabilities arising under the Securities
Act may be permitted to directors, officers and controlling persons of the
Company pursuant to the foregoing provisions, or otherwise, the Company has
been advised that in the opinion of the Securities and Exchange Commission such
indemnification is against public policy as expressed in the Securities Act and
is, therefore, unenforceable. In the event that a claim for indemnification
against such liabilities (other than the payment by the Company of expenses
incurred or paid by a director, officer or controlling person of the Company in
the successful defense of any action, suit or proceeding) is asserted by such
director, officer or controlling person in connection with the securities being
registered, the Company will, unless in the opinion of its counsel the matter
has been settled by controlling precedent, submit to a court of appropriate
jurisdiction the question whether such indemnification by it is against public
policy as expressed in the Securities Act and will be governed by the final
adjudication of such issue.

                                      II-2
<PAGE>

                                   SIGNATURES

   Pursuant to the requirements of the Securities Act of 1933, Monsanto Company
certifies that it has duly caused this Amendment No. 4 to the Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of St. Louis, State of Missouri, on January 25, 2000.

                                          Monsanto Company

                                            /s/ Gary L. Crittenden
                                          By: _________________________________
                                          Name: Gary L. Crittenden
                                          Title:Senior Vice President and
                                          Chief Financial Officer (Principal
                                          Financial Officer)

   Pursuant to the requirements of the Securities Act of 1933, this Amendment
No. 4 to the Registration Statement has been signed by the following persons in
the capacities indicated on the dates indicated:

<TABLE>
<CAPTION>
             Signature                           Title                    Date
             ---------                           -----                    ----
<S>                                  <C>                           <C>
                 *                   Chairman and Director          January 25, 2000
____________________________________ (Principal Executive
         Robert B. Shapiro           Officer)

     /s/ Gary L. Crittenden          Senior Vice President and      January 25, 2000
____________________________________ Chief Financial Officer
         Gary L. Crittenden          (Principal Financial
                                     Officer)

                 *                   Vice President and             January 25, 2000
____________________________________ Controller (Principal
          Richard B. Clark           Accounting Officer)

                 *                   Director                       January 25, 2000
____________________________________
       Richard U. De Schutter

                 *                   Director                       January 25, 2000
____________________________________
           Michael Kantor

                 *                   Director                       January 25, 2000
____________________________________
         Gwendolyn S. King

                 *                   Director                       January 25, 2000
____________________________________
            Philip Leder

                 *                   Director                       January 25, 2000
____________________________________
        Jacobus F. M. Peters

                 *                   Director                       January 25, 2000
____________________________________
            John S. Reed

                 *                   Director                       January 25, 2000
____________________________________
           John E. Robson

                 *                   Director                       January 25, 2000
____________________________________
       William D. Ruckelshaus

                 *                   Director                       January 25, 2000
____________________________________
       Hendrik A. Verfaillie
</TABLE>

* Sonya M. Davis, by signing her name hereto, does sign this document on behalf
of the above noted individuals, pursuant to powers of attorney duly executed by
such individuals which have been filed as an Exhibit to this Registration
Statement.

                                               /s/ Sonya M. Davis
                                          -------------------------------------
                                                   Sonya M. Davis
                                                    Attorney-in-Fact

                                      II-3
<PAGE>

                                 EXHIBIT INDEX

   The following documents are filed herewith or incorporated herein by
reference.

<TABLE>
<CAPTION>
 Exhibit
 Number                          Description of Exhibits
 -------                         -----------------------
 <C>     <S>
  3.1*   Restated Certificate of Incorporation of the Company as of October 28,
         1997 (incorporated herein by reference to Exhibit 3(i) of the
         Company's Form 10-Q for the quarter ended September 30, 1997)

  3.2    By-Laws of the Company, as amended, effective December 10, 1999
         (incorporated herein by reference to Exhibit 3.2 of the Company's Form
         10-K/A for the year ended December 31, 1998 (File No. 1-2516))

  4.1*   Indenture dated as of December 1, 1998 between the Company and The
         Bank of New York as Trustee, providing for Issuance of Debt Securities
         in Series (incorporated herein by reference to Exhibit 4.7 of the
         Company's Current Report on Form 8-K as filed with the Commission on
         December 14, 1998 (File No. 1-2516))

  4.2*   Form of Officer's Certificate, establishing the terms of the Company's
         5.375% Notes due 2001, 5.750% Notes due 2005, 5.875% Notes due 2008,
         6.500% Debentures due 2018 and 6.600% Debentures due 2028

  4.3*   Registration Rights Agreement, dated as of December 9, 1998, by and
         among the Company, Salomon Smith Barney Inc. and Goldman, Sachs & Co.
  5.1*   Opinion of Winston & Strawn
 12.1    Computation of Ratios of Earnings to Fixed Charges (for last five
         fiscal years) (incorporated herein by reference to Exhibit 99 of the
         Company's Form 10-Q/A for the quarter ended September 30, 1999 (File
         No. 1-2516))
 23.1    Consent of Deloitte & Touche LLP
 23.2*   Consent of Winston & Strawn (included in Exhibit 5.1)
 23.3    Consent of Arthur Andersen LLP
 23.4    Consent of PricewaterhouseCoopers LLP
 24.1*   Powers of Attorney
 24.2*   Powers of Attorney
 25.1*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.375% Notes due 2001
 25.2*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.750% Notes due 2005
 25.3*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 5.875% Notes due 2008
 25.4*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 6.500% Debentures due 2018
 25.5*   Form T-1 Statement of Eligibility under the Trust Indenture Act of
         1939 of The Bank of New York relating to the Indenture and the
         issuance of the Company's 6.600% Debentures due 2028
 99.1*   Form of Letter of Transmittal
 99.2*   Form of Notice of Guaranteed Delivery
 99.3*   Form of Tender Instructions
</TABLE>
- --------
 * Previously filed.

                                      II-4

<PAGE>

                                                                    Exhibit 23.1
                                                                    ------------

INDEPENDENT AUDITORS' CONSENT

Monsanto Company:

We consent to the incorporation by reference in this Registration Statement of
Monsanto Company on Amendment No. 4 to Form S-4 of our report dated February 26,
1999 (December 29, 1999 as to the Subsequent Events and Discontinued Operations
Notes), incorporated by reference in the amended annual report on Form 10-K/A of
Monsanto Company for the year ended December 31, 1998 and to the reference to us
under the heading "Experts" in the Prospectus, which is part of this
Registration Statement.


/s/ Deloitte & Touche LLP

St. Louis, Missouri
January 21, 2000

<PAGE>

                                                                    Exhibit 23.3
                                                                    ------------

                   Consent of Independent Public Accountants


     As independent public accountants, we hereby consent to the incorporation
by reference in this Monsanto Company registration statement of our report
(relating to the financial statements of DEKALB Genetics Corporation) dated
October 2, 1998, included in Monsanto Company's Form 8-K/A filed on January 25,
2000 and to all references to our Firm included in this Monsanto Company
registration statement.


/s/ Arthur Andersen LLP

Chicago, Illinois
January 25, 2000

<PAGE>
                                                                    Exhibit 23.4
                                                                    ------------

                      Consent of Independent Accountants

We hereby consent to the incorporation by reference in this Registration
Statement on Form S-4 of Monsanto Company of our report dated February 10, 1999,
except as to Note 1 relating to the pooling of interests with SUGEN, Inc. which
is as of August 31, 1999, which appears in Monsanto Company's Current Report on
Form 8-K dated January 25, 2000. We also consent to the reference to us under
the heading "Experts" in such Registration Statement.



/s/ PricewaterhouseCoopers LLP


Chicago, Illinois
January 25, 2000


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