
<PAGE>
 
                                          File No. 333-______________

   As filed with the Securities and Exchange Commission on December 30, 1998
   --------------------------------------------------------------------------


                       SECURITIES AND EXCHANGE COMMISSION
                            Washington, D.C.  20549
                   _________________________________________

                                    FORM S-8
                             REGISTRATION STATEMENT
                        UNDER THE SECURITIES ACT OF 1933
                   _________________________________________

                                 CHEMFIRST INC.
               (Exact name of issuer as specified in its charter)

               MISSISSIPPI                                 64-0354930
       (State of Incorporation)                   (I.R.S. Employer ID Number)

            700 NORTH STREET
          JACKSON, MISSISSIPPI                                  39202
(Address of Principal Executive Offices)                      (Zip Code)


                  CHEMFIRST INC. 1998 LONG-TERM INCENTIVE PLAN
                            (Full Title of the Plan)

                          JAMES L. MCARTHUR, SECRETARY
                                 CHEMFIRST INC.
                                 P. O. BOX 1249
                        JACKSON, MISSISSIPPI  39215-1249
                                 (601) 948-7550
           (Name, address and telephone number of agent for service)


<TABLE>
<CAPTION>
                                      CALCULATION OF REGISTRATION FEE
 
    Title of               Amount            Proposed           Proposed Maximum      Amount of
  Securities to             to be       Maximum Offering            Aggregate        Registration
  be Registered           Registered     Price Per Share(1)       Offering Price          Fee
 ---------------         ------------   -------------------    -------------------   -------------
<S>                       <C>            <C>                    <C>                  <C>              
Common Stock                950,000          $18.6563               $17,723,485          $4,927.13   
par value $1.00             shares(2)                                              
                                                                                   
Phantom Share Units        maximum of           N/A                      N/A                 N/A
                         950,000 units (3)
</TABLE>

(1)  Estimated solely for calculation of the registration fee pursuant to Rule
     457(h).
(2)  Subject to anti-dilution increases permitted by Rule 416.
(3)  Maximum number of share units is equal to the total number of share units
     which would be convertible into 950,000 shares of common stock.
<PAGE>
 
                                    PART II


INFORMATION REQUIRED IN THE REGISTRATION STATEMENT

ITEM 3.   Incorporation of Documents by Reference

The following documents filed with the Commission by ChemFirst Inc. ("the
Company") are incorporated herein by reference:  (1) the Company's Annual Report
on Form 10-K for the year ended December 31, 1997; (2) Proxy Statement for the
Annual Meeting of Stockholders of the Company held on May 27, 1998; (3) the
Company's Quarterly Reports on Form 10-Q for the quarters ended March 31, 1998,
June 30, 1998 and September 30, 1998; and (4) the description of the Company's
Common Stock, par value $1.00, contained in Item 1 of the Company's Registration
Statement on Form 8-A filed on December 9, 1996, (File No. 001-12547) including
any amendment or report filed for the purpose of updating such description.

All documents filed by the Company pursuant to Sections 13(a), 13(c), 14 and
15(d) of the Exchange Act subsequent to the filing hereof and prior to the
filing of a post-effective amendment which indicates that all securities offered
have been sold or which deregisters all securities then remaining unsold, shall
be deemed to be incorporated by reference in this Registration Statement and to
be a part hereof from the date of filing such documents.

For purposes of this Registration Statement, any document or any statement
contained in a document incorporated or deemed to be incorporated herein by
reference shall be deemed to be modified or superseded to the extent that a
subsequently filed document or a statement contained herein or in any other
subsequently filed document which also is or is deemed to be incorporated herein
by reference modifies or supersedes such document or such statement in such
document.  Any statement so modified or superseded shall not be deemed, except
as so modified or superseded, to constitute a part of this Registration
Statement.

ITEM 4.   Common Stock

Not applicable.

          Phantom Share Units

The phantom share units being registered hereby consist of phantom units of
Company common stock or an amount of cash equal to the value of a share of
common stock at a future date.  They are merely a means of recording a
hypothetical investment in Company common stock or a right to receive a cash
payment and are not actual shares of stock.  Phantom share units have none of
the voting or liquidation rights of Company common stock.  Phantom share units
have no dividend rights per se; however, holders of phantom 

                                      -2-
<PAGE>
 
share units will be entitled to certain dividend equivalents provided for by the
Plan. Specifically, on any date when cash dividends are paid on the shares of
Company common stock, each participant credited with phantom share units under
the Plan as of the record date for such dividend will be credited with a number
of additional share units determined by dividing (i) the aggregate dollar amount
of such cash dividends payable in respect of such participant's phantom share
units (determined by multiplying the dollar value of the dividend paid upon a
single share of Company common stock by the number of share units held by such
participant) by (ii) the fair market value of the Company's common stock as of
the record dividend date.

ITEM 5.   Interests of Named Experts and Counsel

Legal matters, other than those relating to tax consequences, in connection with
the securities covered by this Prospectus have been passed upon by J. Steve
Chustz, General Counsel of the Company.  Mr. Chustz also serves as General
Counsel for various subsidiaries of the Company.  As of November 30, 1998, Mr.
Chustz beneficially owned 47,104 shares of the Company's Common Stock which
includes 45,037 shares of the Company's Common Stock which Mr. Chustz has the
right to acquire through the exercise of Non-Qualified Stock Options. Also as of
November 30, 1998, Mr. Chustz beneficially owns 1,769 of the Company's share
units.

The consolidated financial statements and financial statement schedules of the
Company and subsidiaries as of December 31, 1997 and 1996 and for each of the
years in the three-year period ended December 31, 1997, which are incorporated
herein by reference, have been incorporated herein in reliance upon the report,
also incorporated herein by reference, of KPMG Peat Marwick LLP, independent
certified public accountants, and upon the authority of said firm as experts in
accounting and auditing.  To the extent that KPMG Peat Marwick LLP audits and
reports on financial statements of the Company and subsidiaries issued at future
dates, and consents to the use of their reports thereon, such financial
statements also will be incorporated herein by reference in reliance upon their
reports and said authority.

ITEM 6.   Indemnification of Directors and Officers
 
Subarticle E of Article 8 of the Mississippi Business Corporation Act ("MBCA")
empowers a Mississippi corporation to indemnify against liability an individual
who is made a party to any threatened, pending or completed action, suit or
proceeding, whether civil, criminal, administrative or investigative, formal or
informal (a "Proceeding"), because such person is or was a director of the
corporation. To be eligible for indemnification, the director must have
conducted himself in good faith and in a manner such person reasonably believed
to be in or not opposed to the best interests of the corporation and, with
respect to any criminal action or proceeding, had no reasonable cause to believe
such person's conduct was unlawful. Liability indemnified against includes the
obligation to pay a judgment, settlement, penalty, fine or reasonable expenses
incurred with respect to a Proceeding. The 

                                      -3-
<PAGE>
 
MBCA precludes a corporation from indemnifying a director in connection with a
Proceeding by or in the right of the corporation in which the director was
adjudged liable to the corporation or in connection with any other Proceeding
charging improper personal benefit to a director, whether or not involving
action in the director's official capacity, in which the director was adjudged
liable on the basis that the personal benefit was improperly received by the
director.

Subarticle E further provides that if a director is wholly successful, on the
merits or otherwise, in the defense of any Proceeding to which he was a party
because he is or was a director, the corporation must indemnify him against
reasonable expenses incurred in connection with the Proceeding.  Also, a court
may order a company to indemnify a director if it determines the director is
fairly and reasonably entitled to indemnification in view of all of the relevant
circumstances.  Subarticle E also allows corporations to indemnify officers,
employees or agents to the same extent as directors, and provides for mandatory
or court-ordered indemnification for these persons as described above.  Finally,
the MBCA allows corporations to purchase and maintain insurance on behalf of
directors, officers, employees or agents against liability asserted against or
incurred by them in that capacity or arising from their status as such, whether
or not the corporation would have the power to indemnify such persons against
liability under Subarticle E.

The Company's Bylaws provide for indemnification of Company's officers and
directors to the fullest extent allowed by Mississippi law and further permit
such indemnification with respect to other employees and agents.  The Company
entered into indemnification agreements with certain of its officers and its
directors.  The effect of these agreements is to add a contractual right of
indemnification to the indemnification rights otherwise granted by law.

The Company maintains directors' and officers' liability insurance which
protects each director and officer from certain claims and suits, including
shareholder derivative suits, even where the director may be determined to not
be entitled to indemnification under the MBCA, and claims and suits arising
under the Securities Act of 1933, as amended.  The policy may also afford
coverage under circumstances where the facts do not justify a finding that the
director or officer acted in good faith and in a manner that was in or not
opposed to the interests of the Company.

The foregoing represents a summary of the general effect of the MBCA, the
Company's Amended and Restated Articles of Incorporation and Bylaws and
directors' and officers' liability insurance coverage for purposes of general
description only.
 
ITEM 7.   Exemption from Registration Claimed

Not applicable.

                                      -4-
<PAGE>
 
ITEM 8.   Exhibits


             4.1    ChemFirst Inc. 1998 Long-Term Incentive Plan was included as
                    Appendix A to the Company's Proxy Statement filed in
                    connection with the Annual Meeting of Stockholders held on
                    May 27, 1998, and is incorporated by reference.

             4.2    Amended and Restated Articles of Incorporation of ChemFirst
                    Inc. were filed as Exhibit 3.1 to Amendment No. 1 to the
                    Company's Form S-1 (Registration No. 333-15789) filed on
                    November 18, 1996, and are incorporated herein by reference.

             4.3    Bylaws of ChemFirst Inc., as amended November 24, 1998.

             4.4    Rights Agreement, dated as of October 30, 1996, between the
                    Company and KeyCorp Shareholder Services, Inc. was filed as
                    Exhibit 4 to Amendment No. 1 to the Company's Form S-1
                    (Registration No. 333-15789) filed on November 18, 1996, and
                    is incorporated herein by reference.
                                        
             4.5    First Amendment to Rights Agreement dated effective May 
                    1, 1997 by and among the Company, KeyCorp Shareholder 
                    Services, Inc. and The Bank of New York.
 
             5.1    Opinion of J. Steve Chustz as to legality of securities
                    being registered.
 
            23.1    Consent of J. Steve Chustz is contained within the opinion
                    of counsel filed as Exhibit 5.1.

            23.2    Consent of KPMG Peat Marwick LLP.

            24.1    Power of Attorney by each of the directors of the Company
                    appointing J. Kelley Williams and R. M. Summerford as
                    attorney-in-fact is located at page 8 of this Registration
                    Statement.


ITEM 9.   Undertakings

       The undersigned Registrant hereby undertakes:

       (1) To file, during any period in which offers or sales are being made, a
       post-effective amendment to this registration statement:

       (i)  To include any prospectus required by Section 10(a)(3) of the
            Securities Act of 1933;

                                      -5-
<PAGE>
 
       (ii) To reflect in the prospectus any facts or events arising after the
            effective date of the registration statement (or the most recent
            post-effective amendment thereof) which, individually or in the
            aggregate, represent a fundamental change in the information set
            forth in the registration statement. Notwithstanding the foregoing,
            any increase or decrease in volume of securities offered (if the
            total dollar value of securities offered would not exceed that which
            was registered) and any deviation from the low or high end of the
            estimated maximum offering range may be reflected in the form of
            prospectus filed with the Commission pursuant to Rule 424(b) if, in
            the aggregate, the changes in volume and price represent no more
            than a 20 percent change in the maximum aggregate offering price set
            forth in the "Calculation of Registration Fee" table in the
            effective registration statement;

     (iii)  To include any material information with respect to the plan of
            distribution not previously disclosed in the registration statement
            or any material change to such information in the registration
            statement;

Provided, however, that paragraphs (a)(1)(i) and (a)(1)(ii) do not apply if the
registration statement is on Form S-3, Form S-8 or Form F3 and the information
required to be included in a post-effective amendment by those paragraphs is
contained in periodic reports filed with or furnished to the Commission by the
Registrant pursuant to Section 13 or Section 15(d) of the Securities Exchange
Act of 1934 that are incorporated by reference in the registration statement.

       (2)  That, for the purpose of determining any liability under the
Securities Act of 1933, each such post-effective amendment 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.

       (3)  To remove from registration by means of a post-effective amendment
any of the securities being registered which remain unsold at the termination of
the offering.

The undersigned Registrant hereby undertakes that, for purposes of determining
any liability under the Securities Act of 1933, each filing of the Registrant's
annual report pursuant to Section 13(a) or Section 15(d) of the Securities
Exchange Act of 1934 (and, where applicable, each filing of an employee benefit
plan's annual report pursuant to Section 15(d) of the Securities Exchange Act of
1934) 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.

Insofar as indemnification for liabilities arising under the Securities Act of
1933 may be permitted to directors, officers and controlling persons of the
Registrant pursuant to the foregoing provisions, or otherwise, the Registrant
has been advised that in the opinion of the Securities and Exchange Commission
such indemnification is against public policy as 

                                      -6-
<PAGE>
 
expressed in the Act and is, therefore, unenforceable. In the event that a claim
for indemnification against such liabilities (other than the payment by the
Registrant of expenses incurred or paid by a director, officer or controlling
person of the Registrant 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 Registrant 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 Act and will
be governed by the final adjudication of such issue.

                                      -7-
<PAGE>
 
                                  SIGNATURES


       Pursuant to the requirements of the Securities Act of 1933, the
Registrant certifies that it has reasonable grounds to believe that it meets all
of the requirements for filing on Form S-8 and has duly caused this Registration
Statement to be signed on its behalf by the undersigned, thereunto duly
authorized, in the City of Jackson, State of Mississippi, on November 24, 1998.

                                CHEMFIRST INC.



                                BY:  /s/ J. Kelley Williams
                                    -------------------------------------
                                    J. Kelley Williams, Chairman and
                                     Chief Executive Officer



       Pursuant to the requirements of the Securities Act of 1933, this
Registration Statement has been signed by the following persons in the
capacities and on the dates indicated.

       KNOW ALL MEN BY THESE PRESENTS, that each person whose signature appears
below constitutes and appoints J. Kelley Williams and R. M. Summerford and each
of them, his true and lawful attorneys-in-fact and agents, with full power of
substitution and resubstitution, for him and in his name, place and stead, in
any and all capacities, to sign any or all amendments to this Registration
Statement and to file the same with all exhibits thereto, and other documents in
connection therewith, with the Securities and Exchange Commission, granting unto
said attorneys-in-fact and agents full power and authority to do and perform
each and every act and thing requisite and necessary to be done in and about the
premises, as fully to all intents and purposes as he might or could do in
person, hereby ratifying and confirming all that said attorneys-in-fact and
agents, or their substitutes, may lawfully do or cause to be done by virtue
hereof.


        SIGNATURE                     TITLE                     DATE
        ---------                     -----                     ----
 
/s/ J. Kelley Williams          Chairman of the Board     November 24, 1998
--------------------------      of Directors, 
J. Kelley Williams              Chief Executive Officer
                                and Director
 
/s/ R. M. Summerford            President and              November 24, 1998
--------------------------      Chief Operating Officer 
R. M Summerford              
 

                                      -8-
<PAGE>
 
        SIGNATURE                     TITLE                     DATE
        ---------                     -----                     ----

 
/s/ Max P. Bowman            Vice President, Finance       November 24, 1998
--------------------------   and Treasurer (Chief 
Max P. Bowman                Financial Officer)    
                             
 
/s/ Troy B. Browning         Controller                    November 24, 1998
--------------------------   (Principal Accounting 
Troy B. Browning             Officer)               
                             
 
/s/ Richard P. Anderson      Director                      November 24, 1998
--------------------------
Richard P. Anderson


/s/ Paul A. Becker           Director                      November 24, 1998
--------------------------
Paul A. Becker

 
/s/ James W. Crook           Director                      November 24, 1998
--------------------------
James W. Crook
 

/s/ Michael J. Ferris        Director                      November 24, 1998
--------------------------
Michael J. Ferris
 

/s/ James E. Fligg           Director                      November 24, 1998
--------------------------
James E. Fligg
 

/s/ Robert P. Guyton         Director                      November 24, 1998
--------------------------
Robert P. Guyton
 

/s/ Paul W. Murrill          Director                      November 24, 1998
--------------------------
Paul W. Murrill
 

/s/ William A. Percy, II     Director                      November 24, 1998
--------------------------
William A. Percy, II
 

/s/ Dan F. Smith             Director                      November 24, 1998
--------------------------
Dan F. Smith
 

/s/ Leland R. Speed          Director                      November 24, 1998
--------------------------
Leland R. Speed
 

/s/ R. Gerald Turner         Director                      November 24, 1998
--------------------------
R. Gerald Turner

                                      -9-
<PAGE>
 
                               Index to Exhibits

Exhibit No.     Description
----------      -----------
   4.1          ChemFirst Inc. 1998 Long-Term Incentive Plan was included as
                Appendix A to the Company's Proxy Statement filed in connection
                with the Annual Meeting of Stockholders held on May 27, 1998,
                and is incorporated by reference.
 
   4.2          Amended and Restated Articles of Incorporation of ChemFirst Inc.
                were filed as Exhibit 3.1 to Amendment No. 1 to the Company's
                Form S-1 (Registration No. 333-15789) filed on November 18,
                1996, and are incorporated herein by reference.
 
   4.3          Bylaws of ChemFirst Inc., as amended November 24, 1998.
 
   4.4          Rights Agreement, dated as of October 30, 1996, between the
                Company and KeyCorp Shareholder Services, Inc. was filed as
                Exhibit 4 to Amendment No. 1 to the Company's Form S-1
                (Registration No. 333-15789) filed on November 18, 1996, and is
                incorporated herein by reference.

   4.5          First Amendment to Rights Agreement dated effective May 1, 1997
                by and among the Company, KeyCorp Shareholder Services, Inc. 
                and The Bank of New York.
 
   5.1          Opinion of J. Steve Chustz as to legality of securities being
                registered.
 
  23.1          Consent of J. Steve Chustz is contained within the opinion of
                counsel filed as Exhibit 5.1.
 
  23.2          Consent of KPMG Peat Marwick LLP.
 
  24.1          Power of Attorney by each of the directors of the Company
                appointing J. Kelley Williams and R. M. Summerford as attorney-
                in-fact is located at page 8 of this Registration Statement.
 
