UNITED STATES
                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM 8-K

                                 CURRENT REPORT


                         Pursuant to Section 13 or 15(d)
                     of the Securities Exchange Act of 1934


     Date of Report (Date of earliest event reported):   APRIL 21, 1999
                                                       -----------------

                               INPUT/OUTPUT, INC.
                               ------------------
             (Exact name of registrant as specified in its charter)

                                    DELAWARE
                                    --------
                 (State or other jurisdiction of incorporation)

            1-13402                                         22-2286646          
- ---------------------------------              ---------------------------------
   (Commission File Number)                    (IRS Employer Identification No.)

         11104 WEST AIRPORT BLVD., STAFFORD, TEXAS          77477
- --------------------------------------------------------------------------------
         (Address of principal executive offices)        (Zip Code)

      Registrant's telephone number, including area code   (281) 933-3339
                                                          ----------------------

                                 NOT APPLICABLE
- --------------------------------------------------------------------------------
          (Former name or former address, if changed since last report)



ITEM 5.  OTHER EVENTS.

         On April 21, 1999, SCF-IV, L.P., a limited partnership organized 
under the laws of the State of Delaware ("SCF-IV"), and Input/Output, Inc., a 
Delaware corporation (the "Company") entered into a Purchase Agreement (the 
"Purchase Agreement") pursuant to which SCF-IV agreed to purchase, in a 
privately negotiated transaction, from the Company 40,000 shares of Series B 
preferred stock, par value $0.01 per share (the "Series B Preferred Stock" or 
the "Initial Shares"), of the Company, and the Company granted to SCF-IV an 
option to purchase up to 15,000 additional shares of Series C preferred 
stock, par value $0.01 per share (the "Series C Preferred Stock" or the 
"Option Shares," and together with the Series B Preferred Stock, the 
"Shares"), of the Company. SCF-IV purchased the Initial Shares on May 7, 1999 
(the "Initial Closing Date"). The consideration paid by SCF-IV for the 
Initial Shares was $40,000,000. The net cash proceeds will be used to fund 
the Company's research and development projects, to provide additional 
working capital and for general corporate purposes.

         For a period of ninety days after the Initial Closing Date (the 
"Option Period"), SCF-IV may, at its sole option, purchase up to 15,000 
shares of the Series C Preferred Stock (the "Exercise Notice") under the 
option granted under the Purchase Agreement. The purchase price payable for 
the Option Shares will be $1,000 per share (or an aggregate maximum of 
$15,000,000).

THE PURCHASE AGREEMENT

         Pursuant to the Purchase Agreement, the Company has agreed to use 
its best efforts to take, or cause to be taken, such action as may be 
necessary or advisable to ensure that immediately following the later of (i) 
the Initial Closing Date and (ii) the expiration or termination of the 
applicable waiting period under the Hart-Scott-Rodino Antitrust Improvements 
Act of 1976, as amended (the "HSR Act"), the Company's Board of Directors 
will be increased by one (if necessary) and an individual designated by 
SCF-IV will be elected as a director of the Company in accordance with the 
terms of the Shares as described below.

         In connection with the Purchase Agreement, the Company has amended 
its Rights Agreement with Harris Trust and Savings Bank as rights agent dated 
as of January 17, 1997 (the "Rights Plan"), to provide that neither SCF-IV 
nor any of its affiliates will be deemed to be an "Acquiring Person" within 
the meaning of the Rights Plan by reason of the transactions contemplated by 
the Purchase Agreement. The Company has agreed in the Purchase Agreement that 
it will not further amend the Rights Plan, or adopt any similar agreement, 
that conflicts with, or restricts SCF-IV to a greater extent than the 
provisions contained in the Purchase Agreement. In addition, the Board of 
Directors of the Company has approved SCF-IV and its affiliates becoming an 
"interested stockholder" within the meaning of Section 203 of the Delaware 
General Corporation Law (the "DGCL") and a "Related Person" within the 
meaning of Article THIRTEENTH of the Company's Certificate of Incorporation 
by reason of the acquisition by SCF-IV or any of its affiliates of (i) the 
Shares, (ii) any shares issued to SCF-IV upon conversion of the Shares (the 
"Underlying Shares"), (iii) any shares of Common Stock permitted to be 
acquired by SCF-IV or any of its affiliates in accordance with the limitation 
set forth in the standstill provisions contained in the Purchase Agreement 
(discussed below) or (iv) any

                                  Page 2 of 11



other securities (including shares of Common Stock issuable upon conversion, 
exercise or exchange thereof pursuant to their terms) received by SCF-IV or 
its affiliates pursuant to the Purchase Agreement.

         Under the Purchase Agreement, SCF-IV has agreed that, without the 
prior written consent of the Company, it will not sell or otherwise transfer 
any of the Shares to any other person or entity other than a partner of 
SCF-IV, who, in any event, shall agree to be bound by the transfer 
restrictions contained in the Purchase Agreement. SCF-IV has also agreed 
that, without the prior written consent of the Company, it will not, prior to 
the earlier of (i) the seventh anniversary of the Initial Closing, or (ii) 
the occurrence of a Change of Control (defined as (i) an agreement providing 
for a Business Combination (defined below under "Series B Preferred Stock") 
is approved by the Board of Directors of the Company or a Business 
Combination is consummated, (ii) a tender offer for Common Stock is approved 
or recommended by the Board of Directors of the Company or (iii) there is a 
redemption, repurchase or reacquisition by the Company of rights issued 
pursuant to the Rights Plan or any waiver of the application of the Rights 
Plan to any beneficial owner other than SCF-IV or its affiliates except as 
approved by SCF-IV's representative to the Board of Directors of the 
Company), sell or otherwise transfer the Underlying Shares in a single 
transaction or series of related transactions involving a number of shares in 
the aggregate constituting in excess of 1% of the Company's issued and 
outstanding shares of Common Stock at the time of such sale or transfer 
(based on the Company's most recent report filed with the Securities and 
Exchange Commission ("SEC") under the Securities Exchange Act of 1934, as 
amended (the "Exchange Act"), prior to such sale or transfer disclosing such 
number of outstanding shares) to any entity which SCF-IV knows competes to a 
material extent with the Company or is engaged to a material extent in the 
energy services or equipment business, without first offering such shares to 
the Company on the same terms and in the manner set forth in the Purchase 
Agreement (subject to certain exceptions relating to an underwritten public 
offering).

         The Company has agreed that, from and after the date of the Purchase 
Agreement, it will not issue any Permitted Parity Securities (as defined 
below), except for (i) the issuance of the Option Shares to SCF-IV or its 
affiliates pursuant to the Purchase Agreement, or (ii) the issuance of a 
number of shares of Permitted Parity Securities issued at a price of $1,000 
cash per share in a single series within eighteen months of the Initial 
Closing equal to 35,000 less the number of Option Shares purchased under the 
Purchase Agreement (or, if the Option Period shall not yet have expired, less 
the full 15,000 Option Shares); provided that if such Permitted Parity 
Securities are issued after the first anniversary of the Initial Closing, 
then SCF-IV will be entitled to prior written notice of such proposed 
issuance and the terms thereof and shall have a right of first refusal option 
to purchase any or all of such securities on the same terms as offered to the 
proposed purchaser. "Permitted Parity Securities" for this purpose means up 
to 35,000 shares of preferred stock of the Company constituting no more than 
two series of preferred stock, each share of which (i) has a liquidation 
preference of not more than $1,000 per share exclusive of accrued and unpaid 
dividends, (ii) has a dividend rate of not more than one percent per annum, 
(iii) has no more than one vote per share with respect to matters on which it 
votes together with the Series B Preferred Stock and other Permitted Parity 
Securities as a single class and (iv) is PARI PASSU with the Series B 
Preferred Stock with respect to the payment of dividends and the 
distributions upon Liquidation (as defined in the Certificate of Designation 
of Series B Preferred Stock). The Series C Preferred Stock that may be issued 
pursuant

                                  Page 3 of 11


to the Purchase Agreement shall be considered Permitted Parity Securities for 
purposes of the foregoing.

         The Company agrees that if it issues any Permitted Parity Securities 
(other than Option Shares) pursuant to the foregoing that have terms that 
SCF-IV reasonably believes to be more favorable to the purchaser of such 
shares than the terms of the Initial Shares, then SCF-IV shall have the right 
to exchange all of the Shares purchased under the Purchase Agreement (or, if 
such issuance occurs prior to the expiration of the Option Period and the 
Option Closing has not occurred, all of the Shares that may be purchased 
under the Purchase Agreement if SCF-IV agrees to pay the applicable purchase 
price therefor upon such exchange) on a share for share basis.

         By executing the Purchase Agreement, SCF-IV has agreed that, for a 
period commencing on the date of the Purchase Agreement and ending on the 
earlier of the third anniversary of the Initial Closing Date or the 
occurrence of a Change of Control (as defined), it will not:

               (a) directly or indirectly, take any action to acquire, in the
         aggregate, beneficial ownership of more than 4% of the Company's
         outstanding Common Stock or voting stock (based on the Company's most
         recent Exchange Act report filed with the SEC prior to such acquisition
         disclosing such number of outstanding shares), excluding from such
         ownership the Initial Shares and Option Shares or the Underlying Shares
         or any other Common Stock or voting stock acquired directly from the
         Company;

               (b) form or encourage the formation of a "group" within the
         meaning of Section 13(d)(3) of the Exchange Act, to acquire, change or
         influence control of the Company;

               (c) solicit, or participate in any "solicitation" of "proxies"
         or become a "participant" in any "election contest" or consent
         solicitation (as such terms are defined or used under Regulation 14A
         under the Exchange Act) with respect to the Company in opposition to
         the recommendation of a majority of the Board of Directors of the
         Company;

               (d) initiate, propose or otherwise solicit stockholders for
         the approval of, one or more stockholder proposals with respect to the
         Company or induce any person to initiate any stockholder proposal, in
         each case in opposition to the recommendation of a majority of the
         Board of Directors of the Company;

               (e) deposit any voting stock in a voting trust or subject them
         to a voting agreement or other agreement or arrangement with respect to
         the voting of such voting stock, other than any such trust, agreement
         or other arrangement involving no persons other than SCF-IV, its
         partners or affiliates; or

               (f) solicit, propose or negotiate with any other person
         (including the Company) with respect to any form of business
         combination or other extraordinary transaction with the Company or any
         of its subsidiaries, in each case which would result in a Change of
         Control, or solicit, make or propose or negotiate with any other person
         with respect to or announce

                                  Page 4 of 11



         an intent to make, any tender offer or exchange offer for any
         securities of the Company, in each case in opposition to the
         recommendation of a majority of the Board of Directors of the Company,
         or publicly disclose an intent, purpose, plan or proposal with respect
         to the Company, any of its subsidiaries, or any securities or assets of
         the Company, that would violate the provisions of this paragraph (f);

provided, however, that nothing in the foregoing paragraphs (a)-(f) shall be 
deemed to limit in any way (i) the right of SCF-IV to exercise its voting 
rights in any manner it sees fit with respect to the Shares, the Underlying 
Shares or any other shares of voting stock acquired by SCF-IV in accordance 
with the Purchase Agreement, or (ii) the right of any director elected to the 
Board of Directors as a representative of the holders of the Shares to take 
any action he believes necessary to fulfill his fiduciary duties.

         The Company has agreed to indemnify SCF-IV and its affiliates and 
hold SCF-IV and its affiliates harmless from and against any and all 
liabilities, losses, damages, costs and expenses of any kind, which may be 
incurred by SCF-IV or such affiliates as a result of any claims made against 
SCF-IV or such affiliates by any person that relate to or arise out of (i) 
any breach by the Company of any of its representations, warranties or 
covenants contained in the Purchase Agreement or in the agreements related 
thereto, or (ii) any litigation, investigation or proceeding instituted by 
any person with respect to the Purchase Agreement or the Shares or the 
Underlying Shares (excluding, however, any such litigation, investigation or 
proceeding which arises solely from the acts or omissions of SCF-IV or its 
affiliates). Notwithstanding the foregoing, the Company will not be 
responsible for or assume any of the investment risk associated with any 
securities purchased under the Purchase Agreement.

THE SERIES B PREFERRED STOCK

         The holders of Series B Preferred Stock are entitled to receive 
cumulative cash dividends of $10.00 per annum (1% of the liquidation 
preference) for each share of Series B Preferred Stock. The Series B 
Preferred Stock is entitled to a liquidation preference of $1,000.00, plus 
all accrued but unpaid dividends.

         The Series B Preferred Stock is convertible at the holder's option 
after the first to occur of any of the following (the "Initial Conversion 
Date"): (i) the third anniversary of the original date of issuance of such 
Shares (the "Issue Date"), (ii) the approval by the Board of Directors of the 
Company of an agreement relating to a Business Combination or the 
consummation of a Business Combination, (iii) a tender offer for Common Stock 
is approved or recommended by the Board of Directors of the Company or (iv) 
the redemption, repurchase or reacquisition by the Company of rights issued 
pursuant to the Rights Plan or any waiver of the application of the Rights 
Plan to any beneficial owner other than SCF-IV or its affiliates (except as 
approved by SCF-IV's representative on the Board of Directors of the 
Company). After the Initial Conversion Date and prior to the Mandatory 
Conversion Date (defined below), the holders of Series B Preferred Stock will 
be entitled to convert any and all shares of Series B Preferred Stock into a 
number of fully paid and nonassessable shares of Common Stock per share equal 
to, at the option of the holder, one of, or if

                                  Page 5 of 11


not specified by the holder, at the greater of, the following (such amount 
being referred to as the "Conversion Ratio"): (a) $1,000.00 (plus any accrued 
and unpaid dividends through the record date for determining shareholders 
entitled to vote) divided by the conversion price of $8.00 (as adjusted from 
time to time in accordance with the anti-dilution provisions in the Series B 
Preferred Stock's Certificate of Designation) or (b) $1,000.00 increased at a 
rate of eight percent per annum from the Issue Date, compounded quarterly, 
less the amount of cash dividends actually paid through the applicable 
conversion date (the "Adjusted Stated Value"), divided by the average market 
price for the Common Stock during the ten trading day period prior to the 
date of conversion (the "Conversion Ratio").

         On April 20, 1999, the closing sales price per share of the Common 
Stock on the New York Stock Exchange was $6.875.

         A "Business Combination" for this purpose means (i) any 
consolidation or merger of the Company with or into any person or (ii) any 
Change of Control Stock Issuance (defined below), or (iii) the sale, 
assignment conveyance, transfer, lease or other disposition by the Company of 
all or substantially all of its assets followed by a liquidation of the 
Company. A "Change of Control Stock Issuance" means any issuance, in a single 
transaction or series of related transactions, by the Company of shares of 
Common Stock or Common Stock equivalents in connection with the acquisition 
of assets (including cash) or securities by the Company or a subsidiary of 
the Company (including by way of a merger of a subsidiary of the Company with 
or into a person), except where (i) the shareholders of the Company 
immediately prior to such issuance own (in substantially the same proportion 
relative to each other as such shareholders owned the Common Stock or voting 
stock of the Company, as the case may be, immediately prior to such 
consummation) (x) more than 50% of the voting stock of the Company 
immediately after such issuance, and (y) more than 50% of the outstanding 
Common Stock immediately after such issuance, (ii) the members of the Board 
of Directors of the Company immediately prior to entering into the agreement 
relating to such issuance (or if no such agreement is entered into, then 
immediately prior to the consummation of such issuance) constitute at least a 
majority of the Board of Directors of the Company immediately after such 
issuance, with no agreements or arrangements in place immediately after such 
consummation that would result in the members of the Board of Directors of 
the Company immediately prior to the entering into the agreement relating to 
such issuance ceasing to constitute at least a majority of the Board of 
Directors of the Company and (iii) no person or group of persons immediately 
after such issuance is the beneficial owner of 40% or more of the total 
outstanding voting stock of the Company or Common Stock.

         On the fifth anniversary of the Issue Date (the "Mandatory 
Conversion Date"), each outstanding share of Series B Preferred Stock shall, 
without any action on the part of the holder, be converted automatically into 
a number of fully paid and nonassessable shares of Common Stock equal to the 
Conversion Ratio, provided the Shelf Registration Statement (discussed below) 
contemplated by the Registration Rights Agreement is then in effect.

         In the event of a conversion of Series B Preferred Stock pursuant to 
which the Conversion Ratio is determined using clause (b) above, then, 
provided that full cumulative dividends have been

                                  Page 6 of 11


paid or declared and set apart for payment upon all outstanding shares of 
Series B Preferred Stock for all past dividend periods, the Company may 
redeem for cash up to 50% (or such greater percentage as the holders shall 
agree) of the shares of Series B Preferred Stock submitted for conversion at 
a redemption price per share equal to the Adjusted Stated Value (a 
"Redemption Offer"), in lieu of effecting such conversion.

         If the Company issues Common Stock upon conversion of the Series B 
Preferred Stock pursuant to which the Conversion Ratio is calculated pursuant 
to clause (b) of the definition of Conversion Ratio as set forth above, the 
Company shall not be obligated to issue, in the aggregate, a number of shares 
of Common Stock in excess of the NYSE Limitation upon conversion of the 
Series B Preferred Stock. The "NYSE Limitation" shall mean the maximum number 
of shares of Common Stock that could be issued by the Company pursuant to the 
conversion of the Series B Preferred Stock and any substantially similar 
series of Permitted Parity Securities issued to the holder (including any 
Series C Preferred Stock) pursuant to the terms of the Purchase Agreement 
without triggering a requirement to obtain the approval of the Company's 
shareholders of such issuance pursuant to Section 312.03(c) of the New York 
Stock Exchange Listed Company Manual as in effect on the Issue Date. To the 
extent that any shares of Series B Preferred Stock are submitted for 
conversion such that the NYSE Limitation would be exceeded, such excess 
shares shall, in lieu of being converted into Common Stock, be redeemed in 
exchange for a cash payment equal to the Adjusted Stated Value per share.

         Following the expiration or earlier termination of the applicable 
waiting period under the HSR Act (the "HSR Expiration Date"), holders of the 
Series B Preferred Stock will be entitled to vote upon all matters upon which 
the holders of Common Stock and other Permitted Parity Securities are 
entitled to vote. The holders of Series B Preferred Stock, when voting 
together with the Common Stock as a single class, will be entitled to cast a 
number of votes equal to $1,000.00 (plus any accrued and unpaid dividends 
through the record date for determining the shareholders entitled to vote) 
divided by the conversion price of $8.00 (as adjusted from time to time in 
accordance with the Series B Preferred Stock's Certificate of Designation). 
After the HSR Expiration Date, the holders of Series B Preferred Stock, 
voting together as a class with other Permitted Parity Securities, shall be 
entitled to elect one member of the Board of Directors of the Company. The 
consent of the holders of a majority of the Series B Preferred Stock, voting 
together as a class with other Permitted Parity Securities, will be necessary 
to (i) amend, alter or repeal the Series B Preferred Stock Certificate of 
Designation to authorize, create or issue parity securities or senior 
securities, (ii) issue any parity or senior securities or (iii) consummate 
any Business Combination. Further, the consent of the holders of at least a 
majority of the Series B Preferred Stock, voting separately as a single class 
with one vote per share, will be necessary to amend, alter or repeal any of 
the provisions of (i) the Series B Preferred Stock Certificate of Designation 
or any certificate of designation relating to any parity securities, or (ii) 
the Certificate of Incorporation of the Company, so as to adversely affect 
any of the rights, preferences or privileges of the holders of Series B 
Preferred Stock.

THE SERIES C PREFERRED STOCK

         The Series C Preferred Stock, if issued, will have substantially the 
same terms as the Series

                                  Page 7 of 11


B Preferred Stock, except for the determination of the conversion price. The 
conversion price for the Series C Preferred Stock will be equal to 125% of 
the average market price per share of the Common Stock for the ten trading 
days ending on and including the trading day prior to the date on which 
SCF-IV notifies the Company that it wishes to exercise its option to purchase 
some or all of the Series C Preferred Stock; provided, however, that if such 
average is less than $4.80, then the initial conversion price shall be $6.00 
and if such average is higher than $6.80, the initial conversion price shall 
be $8.50, subject to certain anti-dilution adjustments.

THE REGISTRATION RIGHTS AGREEMENT

         The Company has entered into a Registration Rights Agreement with 
SCF-IV pursuant to which the holder or holders of at least 25% of the then 
outstanding Registrable Securities (which subject to certain limitations 
means the Underlying Shares and any other securities issued or issuable with 
respect to the Underlying Shares) at any time on or after the earlier of (i) 
the Initial Conversion Date or (ii) the date sixty days prior to the third 
anniversary of the date of the initial issuance of the Series B Preferred 
Stock, may request a registration of the Company of any or all of SCF-IV's 
Registrable Securities (a "Demand Registration"); provided, however, that the 
number of Registrable Securities to be included in such a Demand Registration 
must be at least 1,000,000 or such lesser number of Registrable Securities as 
have an aggregate market price of at least $10,000,000 as of the date of such 
request. The holders of Registrable Securities will be entitled to request an 
aggregate of two Demand Registrations. The Registration Rights Agreement also 
entitles the holders of Registrable Securities the right to have such 
securities registered whenever the Company proposes to register any 
securities under the Securities Act of 1933, as amended, (a "Piggyback 
Registration"). As contemplated by the Registration Rights Agreement, the 
ability of the holders of Registrable Securities to participate in Piggyback 
Registration will depend on the individual circumstances and the nature of 
the offering at issue. The Registration Rights Agreement obligates the 
Company to file a shelf registration statement (the "Shelf Registration 
Statement") relating to all of the Registrable Securities prior to the 
Mandatory Conversion Date and to cause such Shelf Registration Statement to 
remain in effect for one year (subject to certain suspension rights). The 
registration expenses with relation to Registrable Securities included in a 
Demand Registration will be borne by the holders of Registrable Securities; 
the registration expenses with relation to Registrable Securities included in 
a Piggyback Registration or the Shelf Registration will be borne by the 
Company.

BENEFICIAL OWNERSHIP BY SCF-IV

         According to its Schedule 13D filed with the SEC on April 30, 1999, 
SCF-IV may be deemed to beneficially own 5,000,000 shares of Common Stock 
based on its acquisition of 40,000 shares of Series B Preferred Stock. The 
5,000,000 shares of Common Stock represent shares issuable to SCF-IV upon 
conversion of the Initial Shares, based on the $8.00 fixed conversion price 
described above. Such 5,000,000 shares of Common Stock would constitute 
approximately 9.0% of the issued and outstanding Common Stock of the Company. 
This amount excludes (i) an indeterminate number of additional shares of 
Common Stock that may be acquired by SCF-IV upon conversion of the Initial 
Shares pursuant to market price-based conversions as described above, or in 
respect of accrued and unpaid dividends and (ii) shares of Common Stock that 
may be acquired upon the conversion of the

                                  Page 8 of 11



Option Shares, since the conversion price for such shares has not been 
established.

         The sale of the Series B Preferred Stock and the offer to sell the 
Series C Preferred Stock was made in reliance on the exemption from 
registration provided by Section 4(2) of the Securities Act of 1933, as 
amended.

         The foregoing, to the extent it summarizes terms of agreements or 
documents, is qualified in its entirety by reference to the full text of such 
documents, copies of which have been filed herewith as exhibits and are 
incorporated herein by reference.

                                  Page 9 of 11





ITEM 7.  FINANCIAL STATEMENTS AND EXHIBITS.
            
         4.1*     Certificate of Designation related to Series B Preferred Stock
                  dated May 7, 1999.

         4.2*     Form of Certificate of Designation related to Series C 
                  Preferred Stock.

        10.1*     Purchase Agreement by and between the Company and SCF-IV dated
                  April 21, 1999.

        10.2*     Registration Rights Agreement by and between the Company and 
                  SCF-IV dated May 7, 1999.

        10.3*     First Amendment to Rights Agreement by and between the Company
                  and Harris Trust and Savings Bank as Rights Agent, dated 
                  April 21, 1999.



- ------------------
*  Filed herewith.


                                  Page 10 of 11


                                   SIGNATURES

         Pursuant to the requirements of the Securities Exchange Act of 1934, 
the registrant has duly caused this report to be signed on its behalf by the 
undersigned hereunto duly authorized.

                                       INPUT/OUTPUT, INC.


                                       By:     /s/ W. J. "ZEKE" ZERINGUE
                                              ------------------------------
                                                W. J. "Zeke" Zeringue
                                                Chief Executive Officer
Date: May 7, 1999



                                  Page 11 of 11