SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549

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

                                    FORM 10-K

   (mark one)
   [ X ] Annual Report Pursuant to Section 13 or 15(d) of the Securities
         Exchange Act of 1934 for the fiscal year ended January 3, 1998
   [   ] Transition Report Pursuant to Section 13 or 15(d) of the Securities
         Exchange Act of 1934

                          Commission file number 1-9786

                         THERMO INSTRUMENT SYSTEMS INC.
             (Exact name of Registrant as specified in its charter)
   Delaware                                                         04-2925809
   (State or other jurisdiction of                            (I.R.S. Employer
   incorporation or organization)                          Identification No.)

   860 West Airport Freeway
   Suite 301
   Hurst, Texas                                                          76054
   (Address of principal executive offices)                         (Zip Code)
       Registrant's telephone number, including area code: (617) 622-1000

           Securities registered pursuant to Section 12(b) of the Act:

        Title of each class        Name of each exchange on which registered
    ----------------------------   -----------------------------------------
    Common Stock, $.10 par value            American Stock Exchange
           Securities registered pursuant to Section 12(g) of the Act:
                                      None

   Indicate by check mark whether the Registrant (1) has filed all reports
   required to be filed by Section 13 or 15(d) of the Securities Exchange Act
   of 1934 during the preceding 12 months, and (2) has been subject to the
   filing requirements for at least the past 90 days. Yes [ X ] No [  ]

   Indicate by check mark if disclosure of delinquent filers pursuant to Item
   405 of Regulation S-K is not contained herein, and will not be contained,
   to the best of the Registrant's knowledge, in definitive proxy or
   information statements incorporated by reference into Part III of this Form
   10-K or any amendment to this Form 10-K. [  ]
   The aggregate market value of the voting stock held by nonaffiliates of the
   Registrant as of January 30, 1998, was approximately $622,251,000.

   As of January 30, 1998, the Registrant had 122,045,212 shares of Common
   Stock outstanding.

                       DOCUMENTS INCORPORATED BY REFERENCE

   Portions of the Registrant's Annual Report to Shareholders for the year
   ended January 3, 1998, are incorporated by reference into Parts I and II.
   Portions of the Registrant's definitive Proxy Statement for the Annual
   Meeting of Shareholders to be held on June 1, 1998, are incorporated by
   reference into Part III.
PAGE
<PAGE>
                                     PART I

    Item 1. Business
            --------

    (a) General Development of Business.
        -------------------------------
        Thermo Instrument Systems Inc. (the Company or the Registrant) is a
    worldwide leader in the development, manufacture, and marketing of
    analytical instruments used to identify and quantify complex molecular
    compounds and elements in gases, liquids, and solids. The Company also
    provides instruments used to monitor radioactivity and air pollution, and
    to control, image, inspect, and measure various industrial processes and
    life-sciences phenomena. Through its 77%-owned ThermoSpectra Corporation
    subsidiary, the Company develops, manufactures, and markets imaging and
    inspection, temperature-control, and test and measurement instruments.
    The Company's 88%-owned ThermoQuest Corporation subsidiary develops,
    manufactures, and sells mass spectrometers, liquid chromatographs, and
    gas chromatographs for the pharmaceutical, environmental, and industrial
    marketplaces. These analytical instruments are used in the quantitative
    and qualitative chemical analysis of organic and inorganic compounds at
    ultra-trace levels of detection. In addition, the Company manufactures
    scientific equipment for the preparation and preservation of chemical
    samples, and consumables for the chromatography industry. Through its
    91%-owned Thermo Optek Corporation subsidiary, the Company develops,
    manufactures, and markets analytical light-based instruments that are
    used in the quantitative and qualitative chemical analysis of elements
    and molecular compounds. Through its 70%-owned Thermo BioAnalysis
    Corporation subsidiary, the Company develops, manufactures, and markets
    instruments, consumables, and information management systems used in
    biochemical research and production, as well as in clinical diagnostics.
    The Company's 60%-owned Metrika Systems Corporation subsidiary develops,
    manufactures, and markets on-line process optimization systems that
    employ proprietary, ultra-high-speed advanced scientific measurement
    technologies for applications in raw-materials analysis and
    finished-materials quality control. The Company's 78%-owned Thermo Vision
    Corporation subsidiary designs, manufactures, and markets a diverse array
    of photonics products that are used in a number of industries for
    research, testing, detecting, and manufacturing applications. The
    Company's 87%-owned, privately held ONIX Systems Inc. subsidiary designs,
    develops, markets, and services sophisticated field measurement
    instruments and on-line sensors. Through its wholly owned subsidiaries,
    the Company also manufactures monitoring instruments to detect and
    monitor environmental pollutants and provides clinical laboratory
    equipment and consumables that assist in the diagnosis of various
    diseases.

        The Company has adopted a strategy of spinning out certain of its
    businesses into separate subsidiaries and having these subsidiaries sell
    a minority interest to outside investors. The Company believes that this
    strategy provides additional motivation and incentives for the management
    of the subsidiaries through the establishment of subsidiary-level stock
    option incentive programs, as well as capital to support the
    subsidiaries' growth. During 1997*, Metrika Systems and Thermo Vision 

    * References to 1997, 1996, and 1995 herein are for the fiscal years
      ended January 3, 1998, December 28, 1996, and December 30, 1995,
      respectively.
                                        2PAGE
<PAGE>
    sold shares of their common stock in initial public offerings and ONIX
    and ThermoQuest sold shares of their common stock in private placements
    for aggregate net proceeds of $86.3 million. See Note 11 to Consolidated
    Financial Statements in the Registrant's 1997 Annual Report to
    Shareholders for a description of the issuance of stock by the Company's
    subsidiaries.

        The Company historically has expanded both through the acquisition of
    companies and product lines and through internal development of new
    products and technologies. During the past several years, the Company has
    completed a number of complementary acquisitions that have provided
    additional technologies, specialized manufacturing or product development
    expertise, and broader capabilities in marketing and distribution. In
    March 1997, the Company acquired 95% of Life Sciences International PLC
    (Life Sciences), a London Stock Exchange-listed company. Subsequently,
    the Company acquired the remaining shares of Life Sciences' capital
    stock. The aggregate purchase price for Life Sciences was approximately
    $442.8 million, net of $55.8 million of cash acquired. The purchase price
    includes the repayment of $105.0 million of Life Sciences' bank debt.
    Life Sciences manufactures laboratory science equipment, appliances,
    instruments, consumables, and reagents for the research, clinical, and
    industrial markets.

        The Company was incorporated in Delaware in May 1986 as a wholly
    owned subsidiary of Thermo Electron Corporation to operate the
    instruments businesses that were previously conducted by several Thermo
    Electron subsidiaries. As of January 3, 1998, Thermo Electron owned
    99,819,138 shares, or 82%, of the Company's outstanding common stock.
    Thermo Electron provides analytical and monitoring instruments;
    biomedical products including heart-assist devices, respiratory-care
    equipment, and mammography systems; paper recycling and papermaking
    equipment; alternative-energy systems; industrial process equipment; and
    other specialized products. Thermo Electron also provides industrial
    outsourcing, laboratory, and metallurgical services, and conducts
    advanced-technology research and development.

        Thermo Electron intends for the foreseeable future to maintain at
    least 80% ownership of the Company, so that it may continue to file
    consolidated U.S. federal and certain state income tax returns with the
    Company. This may require the purchase by Thermo Electron of additional
    shares of common stock and/or convertible debentures of the Company from
    time to time as the number of outstanding shares of the Company
    increases. These and any other purchases may be made either in the open
    market or directly from the Company or pursuant to conversions of the
    Company's 3 3/4% senior convertible note due 2000 held by Thermo
    Electron. See Notes 5 and 7 to Consolidated Financial Statements in the
    Registrant's 1997 Annual Report to Shareholders for a description of the
    Company's outstanding stock options and convertible obligations. During
    1997, Thermo Electron purchased 739,600 shares of the Company's common
    stock in the open market at a total cost of $24.1 million.


                                        3PAGE
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    Forward-looking Statements

        Forward-looking statements, within the meaning of Section 21E of the
    Securities Exchange Act of 1934, are made throughout this Annual Report
    on Form 10-K. For this purpose, any statements contained herein that are
    not statements of historical fact may be deemed to be forward-looking
    statements. Without limiting the foregoing, the words "believes,"
    "anticipates," "plans," "expects," "seeks," "estimates," and similar
    expressions are intended to identify forward-looking statements. There
    are a number of important factors that could cause the results of the
    Company to differ materially from those indicated by such forward-looking
    statements, including those detailed under the heading "Forward-looking
    Statements" in the Registrant's 1997 Annual Report to Shareholders, which
    statements are incorporated herein by reference.

    (b) Financial Information About Industry Segments.
        ---------------------------------------------

        The Company operates in one business segment: the developing,
    manufacturing, marketing, and servicing of analytical instruments and
    software used for the identification and quantification of complex
    molecular compounds and elements in gases, liquids, and solids. Uses
    include pharmaceutical drug research and clinical diagnostics, monitoring
    and measuring environmental pollutants, industrial inspection, and test
    and control for quality assurance and productivity improvement. In
    addition, the Company develops, manufactures, markets, and services
    equipment for the measurement, preparation, storage, and automation of
    sample materials, and photonics products and vacuum components for
    original equipment manufacturers.

    (c) Description of Business.
        -----------------------

        Principal Products and Services

        The Company manufactures and markets instruments that employ a
    variety of advanced analytical techniques to determine the composition,
    structure, and physical properties of natural and synthetic substances.
    The Company's instruments are used for analysis, test and measurement,
    environmental and nuclear monitoring, process control, and in life
    sciences applications. Revenues from analytical and test and measurement
    instruments were $1,084.3 million, $821.6 million, and $537.5 million in
    1997, 1996, and 1995, respectively.

        ThermoSpectra develops, manufactures, and markets precision imaging
    and inspection, temperature-control, and test and measurement
    instruments. These instruments are generally combined with proprietary
    operations and analysis software to provide industrial and research
    customers with integrated systems that address their specific needs.

        ThermoQuest is a leading provider of mass spectrometers, liquid
    chromatographs, and gas chromatographs for the pharmaceutical,
    environmental, and industrial marketplaces. These analytical instruments
    are used in the quantitative and qualitative chemical analysis of organic
    and inorganic compounds at ultratrace levels of detection. ThermoQuest

                                        4PAGE
<PAGE>
    also supplies scientific equipment for the preparation and preservation
    of chemical samples, and consumables for the chromatography industry.

        Thermo Optek is a worldwide leader in the development, manufacture,
    and marketing of analytical instruments that use a range of light- and
    energy-based techniques. Thermo Optek's instruments are used in the
    quantitative and qualitative chemical analysis of elements and molecular
    compounds in a variety of solids, liquids, and gases.

        Thermo BioAnalysis develops, manufactures, and markets instruments,
    consumables, and information-management systems used in biochemical
    research and production, as well as in clinical diagnostics. Thermo
    BioAnalysis focuses on three principal product areas: life sciences
    instrumentation and consumables, information-management systems, and
    health physics instrumentation.

        Metrika Systems manufactures process optimization systems that
    provide on-line, real-time analysis of the elemental composition of bulk
    raw materials in basic-materials production processes, including coal,
    cement, and minerals. In addition, Metrika Systems manufactures
    industrial gauging and process-control instruments and systems used
    principally by manufacturers of finished web materials, such as sheet
    metal, rubber, and plastic foils, to measure and control parameters such
    as thickness and coating weight of such materials.

        Thermo Vision, which became a public subsidiary of the Company in
    December 1997, designs, manufactures, and markets a diverse array of
    photonics (light-based) products, including optical components, imaging
    sensors and systems, lasers, optically based instruments, opto-
    electronics, and fiber optics. These products are used in applications
    including medical diagnostics, semiconductor production, X-ray imaging,
    physics research, and telecommunications.

        ONIX Systems, a privately held subsidiary of the Company, designs,
    develops, markets, and services sophisticated field measurement
    instruments and on-line sensors for process-control industries,
    particularly oil and gas. Systems provide real-time data collection,
    analysis, and local control functions regarding the flow, level, density,
    or composition of a particular material.

        Thermo Instrument also has wholly owned businesses, including the
    Life Sciences Clinical Instrument Division, which provides an array of
    clinical laboratory equipment and consumables, and Thermo Monitoring
    Instruments, which produces instruments and complete systems for
    detecting and monitoring environmental pollutants from industrial and
    mobile sources, and for detecting radioactive contamination.

        Backlog

        The Company's backlog of firm orders was $298.9 million as of January
    3, 1998, and $266.6 million as of December 28, 1996, The Company
    anticipates that substantially all of the backlog as of January 3, 1998,
    will be shipped or completed during 1998. Certain of such orders are
    cancellable by the customer upon payment of a cancellation charge. The

                                        5PAGE
<PAGE>
    Company does not believe that the level of, or changes in the level of,
    its backlog is necessarily indicative of intermediate or long-term trends
    in its business.

        Competition

        The Company generally competes on the basis of technical advances
    that result in new products and improved price/performance ratios,
    reputation among customers as a quality leader for products and services,
    and active research and application-development programs. To a lesser
    extent, the Company competes on the basis of price.

        In many markets, the Company competes with large analytical-
    instrument companies such as Hewlett-Packard Co., Perkin-Elmer
    Corporation, Varian Associates, Inc., and Hitachi, Ltd. Certain products
    manufactured by the Company also compete with products sold by numerous
    smaller, specialized firms.

        ThermoSpectra competes in each of its markets primarily on the basis
    of technical advances that result in new products and improved
    price/performance ratios and reputation among customers as a quality
    leader for products and services. To a lesser extent, ThermoSpectra
    competes on the basis of price. The market for digital oscillographic
    recorders is characterized by competition among a number of competitors,
    including Astro-Med, Inc. and Yokogawa Electric Corporation. The
    general-purpose digital storage oscilloscope market is dominated by
    Tektronix, Inc. and Hewlett-Packard. ThermoSpectra competes in the
    high-end of the X-ray microanalysis market on the basis of quality and
    technology, and in the mid-end of the X-ray microanalysis market on the
    basis of quality, performance, and price. The main competitor in this
    segment is Link Analytical Limited, a wholly owned subsidiary of Oxford
    Instruments plc. In the X-ray inspection market, ThermoSpectra competes
    with smaller companies in the manual segment of the market, and primarily
    with Four Pi Systems, a subsidiary of Hewlett-Packard, in the automated
    segment. In the digital-video segment of the confocal microscopy market,
    ThermoSpectra competes primarily with Nikon Inc., as well as Bio-Rad
    Laboratories, Inc. ThermoSpectra competes in the scanning-probe
    microscope market on the basis of quality, performance, and, to a lesser
    extent, price. The dominant competitor in this market is Digital
    Instruments, Inc. In the temperature-control market, the Company competes
    primarily on the basis of performance, price, and customer service. The
    main competitors in this market are Lauder and Jalabo.

        ThermoQuest competes in each of its markets primarily on technical
    performance, customer service and support and, to a lesser extent, price.
    ThermoQuest's principal competitors in the mass spectrometry market
    include Hewlett-Packard, Waters Instruments Inc. (MicroMass), Shimadzu
    Corporation, and Perkin-Elmer. ThermoQuest's principal competitors in the
    liquid chromatography market include Waters, Hewlett-Packard, Shimadzu,
    and Perkin-Elmer. In the gas chromatography market, ThermoQuest competes
    with numerous companies including Hewlett-Packard, Varian, Perkin-Elmer,
    and Shimadzu. ThermoQuest's principal competitors in the sample
    preparation and preservation markets include Jouan S.A., NuAire, Sanyo
    Electric, and Labconco. ThermoQuest's principal competitors in the

                                        6PAGE
<PAGE>
    chromatography consumables market include Waters, Merck, Phenomenex, and
    numerous regional suppliers.

        Thermo Optek competes in each of its markets primarily on
    performance, reliability, customer service, and price. In the market for
    AE and AA spectrometers and ICP/MS instruments, Thermo Optek competes
    primarily with Perkin-Elmer and, to a lesser extent, Varian. Thermo Optek
    competes in the arc/spark market primarily with Spectro. In the FT-IR and
    FT-Raman markets, Thermo Optek competes primarily with Perkin-Elmer and
    Bio-Rad. Thermo Optek entered the market for UV/Vis instruments with its
    acquisition of Unicam in 1995. The primary competitor in this market is
    Perkin-Elmer.

        In life sciences instrumentation, including molecular interaction,
    MALDI-TOF mass spectrometry, DNA amplification, and capillary
    electrophoresis, Thermo BioAnalysis competes primarily on the basis of
    technological innovation, performance, flexibility, and price. Major
    competitors in this category include Perkin-Elmer and PerSeptive
    Biosystems, Inc., which is now a subsidiary of Perkin-Elmer. In
    information management systems, Thermo BioAnalysis competes in the
    high-end LIMS and CDS markets primarily on function, flexibility,
    technical sophistication, customization, and price. Major competitors
    include Hewlett-Packard, Perkin-Elmer, and Waters.

        Metrika Systems competes primarily on the basis of performance and,
    to a lesser extent, price in the on-line coal, cement, and mineral
    analysis markets. Scantech Limited is the Company's primary competitor in
    the on-line coal and cement analysis market. Amdel of Australia is the
    Company's principal competitor in the on-line minerals analysis market.
    The market for solids and multiphase analyzers for process control is
    generally fragmented. Competition in the thickness-gauging business is
    highly fragmented with numerous competitors competing in various end-use
    market segments. As a result, competition varies according to the end-use
    segment. Metrika Systems competes primarily on the basis of quality,
    performance, and price.

        Thermo Vision competes primarily on the basis of technical
    suitability, product performance, reliability, and price, and its
    principal competitors include Melles-Griot, Inc.; Optical Coating
    Laboratory, Inc.; Newport Corporation; Coherent, Inc.; Corning OCA
    Corporation; the Bicron Business Unit of Saint-Gobain Industrial Ceramics,
    Inc.; UDT Sensors, Inc.; and Hamamatsu Corporation, a unit of Hamamatsu
    Photonics KK.

        The Company competes in the field measurement instruments and sensors
    market primarily on quality and reliability, technical features,
    accuracy, ease of use, price, and reputation for after market service.
    The field measurement instruments and sensors market segment of the
    process control market is highly fragmented and intensely competitive,
    and the Company expects competition to continue to increase. ONIX Systems
    competes with a few large competitors, including Fisher-Rosemount, a
    division of Emerson Electric Co., Inc.; Elsag-Bailey Process Automation
    N.V., affiliates of ABB Asea Brown Boveri (Holding) Ltd.; and Yokogawa,
    in each of its product areas and with many companies within specific

                                        7PAGE
<PAGE>
    industries. As the technologies utilized within the process measurement
    industry continue to develop, ONIX Systems expects to face increasing
    competition from emerging competitors.

        The Company is a leading manufacturer of ambient air-monitoring
    instruments and a major manufacturer of source monitoring and
    worker-safety monitoring instruments. The Company competes in these
    markets on the basis of technical performance and reliability, as well as
    customer service. The Company's principal competitors include Monitor
    Labs Incorporated, Advanced Pollution Instruments, and Mine Safety
    Appliances Co.

        The Company has a relatively small presence within the large and
    varied process-control marketplace, which is extremely fragmented and is
    comprised of several large companies, including Fisher-Rosemount, Elsag
    Bailey, and Honeywell Process Control, as well as numerous smaller
    companies. The Company competes in this market primarily on the basis of
    technical performance, customer service, and reliability.

        Environmental Protection Regulations

        The Company believes that compliance by the Company with federal,
    state, and local environmental protection regulations will not have a
    material adverse effect on its capital expenditures, earnings, or
    competitive position.

        Number of Employees

        As of January 3, 1998, the Company employed 9,398 people.

    (d) Financial Information About Exports by Domestic Operations and
        --------------------------------------------------------------
        About Foreign Operations.
        ------------------------

        Financial information about exports by domestic operations and about
    foreign operations is summarized in Note 14 to Consolidated Financial
    Statements in the Registrant's 1997 Annual Report to Shareholders and is
    incorporated herein by reference.

    (e) Executive Officers of the Registrant.
        ------------------------------------
                                            Present Title (Year First Became
        Name                         Age    Executive Officer)
        -------------------------    ---    --------------------------------
        Earl R. Lewis                 54    President and Chief Executive
                                              Officer (1990)
        Denis A. Helm                 59    Senior Vice President (1986)
        Dr. Richard W.K. Chapman      53    Vice President (1994)
        Barry S. Howe                 42    Vice President (1994)
        John N. Hatsopoulos *         63    Chief Financial Officer and 
                                              Senior Vice President (1988)
        Paul F. Kelleher              55    Chief Accounting Officer (1986)

        *John N. Hatsopoulos and George N. Hatsopoulos, a director of the
         Company, are brothers.

                                        8PAGE
<PAGE>
        Each executive officer serves until his successor is chosen or
    appointed by the Board of Directors and qualified or until earlier
    resignation, death, or removal. All executive officers, except Mr. Lewis
    and Dr. Chapman, have held comparable positions for at least five years,
    either with the Company or with its parent company, Thermo Electron. Mr.
    Lewis was named President and Chief Executive Officer of the Company in
    March 1997. Mr. Lewis served as Executive Vice President and Chief
    Operating Officer of the Company from January 1996 through March 1997, as
    a Senior Vice President from January 1994 through January 1996, and as a
    Vice President from March 1990 through January 1994. Dr. Chapman has been
    President and Chief Executive Officer of ThermoQuest since its inception
    in June 1995, and served as President of Finnigan Corporation, a
    subsidiary of ThermoQuest, from 1992 to 1995. Messrs. Lewis, Helm,
    Chapman, and Howe are full-time employees of the Company. Messrs.
    Hatsopoulos and Kelleher are full-time employees of Thermo Electron and
    certain of its subsidiaries, but devote such time to the affairs of the
    Company as the Company's needs reasonably require.

    Item 2. Properties
            ----------

        The Company owns approximately 2,601,000 square feet of office,
    engineering, laboratory, and production space, principally in California,
    Florida, New Mexico, Texas, Wisconsin, Ohio, New Hampshire, New York,
    Massachusetts, the United Kingdom, and Germany, and leases approximately
    2,423,000 square feet of office, engineering, laboratory, and production
    space, under leases expiring from 1998 through 2017, principally in
    California, Massachusetts, Texas, Wisconsin, and the United Kingdom. As
    of January 3, 1998, the Company had an $8.3 million mortgage loan that is
    secured by 200,000 square feet of property in California with a net book
    value of $15.8 million.

        The Company believes that its facilities are in good condition and
    are suitable and adequate for its present operations and that suitable
    space is readily available if any of such leases are not extended. With
    respect to leases expiring in the near future, in the event the Company
    does not renew such leases, the Company believes suitable alternate space
    is available for lease on acceptable terms.

    Item 3. Legal Proceedings
            -----------------

        Not applicable.

    Item 4. Submission of Matters to a Vote of Security Holders
            ---------------------------------------------------

        Not applicable.



                                        9PAGE
<PAGE>
                                     PART II

    Item 5. Market for Registrant's Common Equity and Related Stockholder
            -------------------------------------------------------------
            Matters
            -------

        Information concerning the market and market price for the
    Registrant's common stock, $.10 par value, and dividend policy is
    included under the sections labeled "Common Stock Market Information" and
    "Dividend Policy" in the Registrant's 1997 Annual Report to Shareholders
    and is incorporated herein by reference.

    Item 6. Selected Financial Data
            -----------------------

        The information required under this item is included under the
    sections labeled "Selected Financial Information" and "Dividend Policy"
    in the Registrant's 1997 Annual Report to Shareholders and is
    incorporated herein by reference.

    Item 7. Management's Discussion and Analysis of Financial Condition and
            ---------------------------------------------------------------
            Results of Operations
            ---------------------

        The information required under this item is included under the
    heading "Management's Discussion and Analysis of Financial Condition and
    Results of Operations" in the Registrant's 1997 Annual Report to
    Shareholders and is incorporated herein by reference.

    Item 8. Financial Statements and Supplementary Data
            -------------------------------------------

        The Registrant's Consolidated Financial Statements as of January 3,
    1998, and Supplementary Data are included in the Registrant's 1997 Annual
    Report to Shareholders and are incorporated herein by reference.

    Item 9. Changes in and Disagreements with Accountants on Accounting and
            ---------------------------------------------------------------
            Financial Disclosure
            --------------------

        Not applicable.




                                       10PAGE
<PAGE>
                                    PART III

    Item 10. Directors and Executive Officers of the Registrant
             --------------------------------------------------

        The information concerning directors required under this item is
    incorporated herein by reference from the material contained under the
    caption "Election of Directors" in the Registrant's definitive proxy
    statement to be filed with the Securities and Exchange Commission
    pursuant to Regulation 14A, not later than 120 days after the close of
    the fiscal year. The information concerning delinquent filers pursuant to
    Item 405 of Regulation S-K is incorporated herein by reference from the
    material contained under the heading "Section 16(a) Beneficial Ownership
    Reporting Compliance" under the caption "Stock Ownership" in the
    Registrant's definitive proxy statement to be filed with the Securities
    and Exchange Commission pursuant to Regulation 14A, not later than 120
    days after the close of the fiscal year.

    Item 11. Executive Compensation
             ----------------------

        The information required under this item is incorporated herein by
    reference from the material contained under the caption "Executive
    Compensation" in the Registrant's definitive proxy statement to be filed
    with the Securities and Exchange Commission pursuant to Regulation 14A,
    not later than 120 days after the close of the fiscal year.

    Item 12. Security Ownership of Certain Beneficial Owners and Management
             --------------------------------------------------------------

        The information required under this item is incorporated herein by
    reference from the material contained under the caption "Stock Ownership"
    in the Registrant's definitive proxy statement to be filed with the
    Securities and Exchange Commission pursuant to Regulation 14A, not later
    than 120 days after the close of the fiscal year.

    Item 13. Certain Relationships and Related Transactions
             ----------------------------------------------

        The information required under this item is incorporated herein by
    reference from the material contained under the caption "Relationship
    with Affiliates" in the Registrant's definitive proxy statement to be
    filed with the Securities and Exchange Commission pursuant to Regulation
    14A, not later than 120 days after the close of the fiscal year.










                                       11PAGE
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                                     PART IV

    Item 14. Exhibits, Financial Statement Schedules, and Reports on Form 8-K
             ----------------------------------------------------------------

      (a, d) Financial Statements and Schedules.
             ----------------------------------

             (1) The consolidated financial statements set forth in the list
                 below are filed as part of this Report.

             (2) The consolidated financial statement schedule set forth in
                 the list below is filed as part of this Report.

             (3) Exhibits filed herewith or incorporated herein by reference
                 are set forth in Item 14(c) below.

             List of Financial Statements and Schedules Referenced in this
             -------------------------------------------------------------
             Item 14.
             --------

             Information incorporated by reference from Exhibit 13 filed
             herewith:

                 Consolidated Statement of Income
                 Consolidated Balance Sheet
                 Consolidated Statement of Cash Flows
                 Consolidated Statement of Shareholders' Investment
                 Notes to Consolidated Financial Statements
                 Report of Independent Public Accountants

             Financial Statement Schedules filed herewith:

                 Schedule II: Valuation and Qualifying Accounts

             All other schedules are omitted because they are not applicable
             or not required, or because the required information is shown
             either in the financial statements or in the notes thereto.

         (b) Reports on Form 8-K.
             -------------------

             None.

         (c) Exhibits.
             --------

             See Exhibit Index on the page immediately preceding exhibits.




                                       12PAGE
<PAGE>
                                   SIGNATURES

        Pursuant to the requirements of Section 13 or 15(d) of the Securities
    Exchange Act of 1934, the Registrant has duly caused this report to be
    signed on its behalf by the undersigned, thereunto duly authorized.

    Date: March 11, 1998           THERMO INSTRUMENT SYSTEMS INC.


                                   By: Earl R. Lewis
                                       -----------------------------------
                                       Earl R. Lewis
                                       President and Chief Executive
                                         Officer

        Pursuant to the requirements of the Securities Exchange Act of 1934,
    this report has been signed below by the following persons on behalf of
    the Registrant and in the capacities indicated, as of March 11, 1998.

    Signature                          Title
    ---------                          -----


    By: Earl R. Lewis                 President, Chief Executive Officer,
        ---------------------------    and Director
        Earl R. Lewis                    

    By: John N. Hatsopoulos            Chief Financial Officer,
        ---------------------------      Senior Vice President, and Director
        John N. Hatsopoulos        

    By: Paul F. Kelleher               Chief Accounting Officer
        ---------------------------
        Paul F. Kelleher

    By:                                Director
        ---------------------------
        Frank Borman

    By: George N. Hatsopoulos          Director
        ---------------------------
        George N. Hatsopoulos

    By: Arvin H. Smith                 Chairman of the Board and Director
        ---------------------------
        Arvin H. Smith

    By: Polyvios C. Vintiadis          Director
        ---------------------------
        Polyvios C. Vintiadis




                                       13PAGE
<PAGE>
                    Report of Independent Public Accountants
                    ----------------------------------------

    To the Shareholders and Board of Directors of
    Thermo Instrument Systems Inc.:

        We have audited, in accordance with generally accepted auditing
    standards, the consolidated financial statements included in Thermo
    Instrument Systems Inc.'s Annual Report to Shareholders incorporated by
    reference in this Form 10-K, and have issued our report thereon dated
    February 17, 1998. Our audits were made for the purpose of forming an
    opinion on those statements taken as a whole. The schedule listed in Item
    14 on page 12 is the responsibility of the Company's management and is
    presented for purposes of complying with the Securities and Exchange
    Commission's rules and is not part of the basic consolidated financial
    statements. This schedule has been subjected to the auditing procedures
    applied in the audits of the basic consolidated financial statements and,
    in our opinion, fairly states, in all material respects, the financial
    data required to be set forth therein in relation to the basic
    consolidated financial statements taken as a whole.



                                                 Arthur Andersen LLP



    Boston, Massachusetts
    February 17, 1998











                                       14PAGE
<PAGE>
  SCHEDULE II

                         THERMO INSTRUMENT SYSTEMS INC.
                        Valuation and Qualifying Accounts
                                 (In thousands)


                            Provision
                Balance at    Charged             Accounts            Balance
                 Beginning         to   Accounts   Written             at End
  Description      of Year    Expense  Recovered       Off  Other(a)  of Year
  ---------------------------------------------------------------------------
  Allowance for
    Doubtful Accounts

  Year Ended
   Jan. 3, 1998    $16,981    $ 4,366     $  304   $(4,375)  $5,510   $22,786

  Year Ended
   Dec. 28, 1996   $12,569    $ 2,274     $   69   $(5,015)  $7,084   $16,981

  Year Ended
   Dec. 30, 1995   $ 8,779    $ 2,543     $  191   $(2,942)  $3,998   $12,569

  (a) Includes allowance of businesses acquired during the year as described
      in Note 4 to Consolidated Financial Statements in the Company's 1997
      Annual Report to Shareholders and the effect of foreign currency
      translation.









                                       15PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
      2.1          Asset and Stock Purchase Agreement among the
                   Registrant, Thermo Electron Corporation, and Fisons
                   plc dated March 1, 1995, as amended (filed as
                   Exhibit 2.3 to the Registrant's Annual Report on
                   Form 10-K for the fiscal year ended December 31,
                   1994, and as Exhibit 2 to the Registrant's Quarterly
                   Report on Form 10-Q for the quarter ended September
                   30, 1995 [File No. 1-9786] and incorporated herein
                   by reference). Pursuant to Item 601(b)(2) of
                   Regulation S-K, schedules to this Agreement have
                   been omitted. The Company hereby undertakes to
                   furnish supplementally a copy of such schedules to
                   the Commission upon request.

      2.2          Agreement and Release dated as of December 15, 1997,
                   among Fisons plc, the Registrant and Thermo Electron.

      3.1          Amendment to Restated Certificate of Incorporation
                   of the Registrant (filed as Exhibit 3.1 to the
                   Registrant's Quarterly Report on Form 10-Q for the
                   quarter ended June 29, 1996 [File No. 1-9786] and
                   incorporated herein by reference).

      3.2          By-Laws of the Registrant (filed as Exhibit 3(b) to
                   the Registrant's Annual Report on Form 10-K for the
                   fiscal year ended January 2, 1993 [File No. 1-9786]
                   and incorporated herein by reference).

      4.1          Subordinated Indenture, dated January 15, 1998,
                   among the Registrant, Thermo Electron and Bankers
                   Trust Company as trustee, relating to $250,000,000
                   principal amount of 4% Convertible Subordinated
                   Debentures due 2005 (filed as Exhibit 4.1 to the
                   Registrant's Current Report on Form 8-K filed with
                   the Commission on January 16, 1998, and incorporated
                   herein by reference).

      4.2          Senior convertible note purchase agreement by and
                   between the Registrant and Thermo Electron as of
                   September 15, 1993 (filed as Exhibit 10(a) to the
                   Registrant's Quarterly Report on Form 10-Q for the
                   quarter ended October 2, 1993 [File No. 1-9786] and
                   incorporated by reference).

                   The Registrant hereby agrees, pursuant to Item
                   601(b) (4) (iii) (A) of Regulation S-K, to furnish
                   to the Commission upon request, a copy of each
                   instrument with respect to other long-term debt of
                   the Registrant or its subsidiaries.


                                       16PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     10.1          Amended and Restated Corporate Services Agreement,
                   dated as of January 3, 1993, between Thermo Electron
                   and the Registrant (filed as Exhibit 10(a) to the
                   Registrant's Annual Report on Form 10-K for the
                   fiscal year ended January 2, 1993 [File No. 1-9786]
                   and incorporated herein by reference).

     10.2          Tax Allocation Agreement dated as of May 29, 1986,
                   between Thermo Electron and the Registrant (filed as
                   Exhibit 10(b) to the Registrant's Registration
                   Statement on Form S-1 [Reg. No. 33-6762] and
                   incorporated herein by reference).

     10.3          Thermo Electron Corporate Charter, as amended and
                   restated effective January 3, 1993 (filed as Exhibit
                   10(f) to the Registrant's Annual Report on Form 10-K
                   for the fiscal year ended January 2, 1993 [File No.
                   1-9786] and incorporated herein by reference).

     10.4          Form of Indemnification Agreement with Directors and
                   Officers (filed as Exhibit 10(g) to the Registrant's
                   Annual Report on Form 10-K for the fiscal year ended
                   December 29, 1990 [File No. 1-9786] and incorporated
                   herein by reference).

     10.5          Plan for sale of shares by the Registrant to Thermo
                   Electron (filed as Exhibit 10(dd) to the
                   Registrant's Quarterly Report on Form 10-Q for the
                   quarter ended July 3, 1993 [File No. 1-9786] and
                   incorporated herein by reference).

     10.6          Master Repurchase Agreement dated December 28, 1996,
                   between the Registrant and Thermo Electron (filed as
                   Exhibit 10.6 to the Registrant's Annual Report on
                   Form 10-K for the fiscal year ended December 28,
                   1996 [File No. 1-9786] and incorporated herein by
                   reference).

     10.7          Amended and Restated Master Guarantee Reimbursement
                   and Loan Agreement dated December 2, 1997, by and
                   among the Registrant and Thermo Electron.

     10.8          $30,000,000 Promissory Note dated as of February 13,
                   1996, issued by Thermo BioAnalysis Corporation to
                   Thermo Electron (filed as Exhibit 10.1 to the
                   Registrant's Quarterly Report on Form 10-Q for the
                   quarter ended March 30, 1996 [File No. 1-9786] and
                   incorporated herein by reference).


                                       17PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     10.9          $65,000,000 Promissory Note dated as of April 12,
                   1996, issued by the Registrant to Thermo Electron
                   (filed as Exhibit 10.2 to the Registrant's Quarterly
                   Report on Form 10-Q for the quarter ended March 30,
                   1996 [File No. 1-9786] and incorporated herein by
                   reference).

     10.10         Restated Stock Holdings Assistance Plan and Form of
                   Promissory Note.

     10.11-10.15   Reserved.

     10.16         Deferred Compensation Plan for Directors of the
                   Registrant (filed as Exhibit 10(f) to the
                   Registrant's Registration Statement on Form S-1
                   [Reg. No. 33-6762] and incorporated herein by
                   reference).

     10.17         Directors' Stock Option Plan of the Registrant
                   (filed as Exhibit 10.17 to the Registrant's Annual
                   Report on Form 10-K for the fiscal year ended
                   December 31, 1994 [File No. 1-9786] and incorporated
                   herein by reference).

     10.18         Incentive Stock Option Plan of the Registrant (filed
                   as Exhibit 10(c) to the Registrant's Registration
                   Statement on Form S-1 [Reg. No. 33-6762] and
                   incorporated herein by reference). (Maximum number
                   of shares issuable in the aggregate under this plan
                   and the Registrant's Nonqualified Stock Option Plan
                   is 3,515,625 shares, after adjustment to reflect
                   share increase approved in 1990; 3-for-2 stock
                   splits effected in January 1988, July 1993, and
                   April 1995; and 5-for-4 stock splits effected in
                   December 1995 and October 1997).

     10.19         Nonqualified Stock Option Plan of the Registrant
                   (filed as Exhibit 10(d) to the Registrant's
                   Registration Statement on Form S-1 [Reg. No.
                   33-6762] and incorporated herein by reference).
                   (Maximum number of shares issuable in the aggregate
                   under this plan and the Registrant's Incentive Stock
                   Option Plan is 3,515,625 shares, after adjustment to
                   reflect share increase approved in 1990; 3-for-2
                   stock splits effected in January 1988, July 1993,
                   and April 1995; and 5-for-4 stock splits effected in
                   December 1995 and October 1997).



                                       18PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     10.20         Equity Incentive Plan of the Registrant (filed as
                   Appendix A to the Proxy Statement dated April 27,
                   1993, of the Registrant [File No. 1-9786] and
                   incorporated herein by reference). (Maximum number
                   of shares issuable is 5,039,062 shares, after
                   adjustment to reflect share increase approved in
                   December 1993; 3-for-2 stock splits effected in July
                   1993 and April 1995; and 5-for-4 stock splits
                   effected in December 1995 and October 1997).

     10.21         Finnigan Corporation 1979 Long-term Incentive Stock
                   Option Plan (filed as Exhibit 10.21 to the
                   Registrant's Annual Report on Form 10-K for the
                   fiscal year ended December 31, 1994 [File No.
                   1-9786] and incorporated herein by reference).

     10.22         Former Thermo Environmental Corporation Incentive
                   Stock Option Plan (filed as Exhibit 10(d) to Thermo
                   Environmental's Registration Statement on Form S-1
                   [Reg. No. 33-329] and incorporated herein by
                   reference). (Maximum number of shares issuable in
                   the aggregate under this plan and the Former Thermo
                   Environmental Corporation Nonqualified Stock Option
                   Plan is 1,450,195 shares, after adjustment to
                   reflect share increase approved in 1987; 3-for-2
                   stock splits effected in July 1993 and April 1995;
                   and 5-for-4 stock splits effected in December 1995
                   and October 1997).

     10.23         Former Thermo Environmental Corporation Nonqualified
                   Stock Option Plan (filed as Exhibit 10(e) to Thermo
                   Environmental's Registration Statement on Form S-1
                   [Reg. No. 33-329] and incorporated herein by
                   reference). (Maximum number of shares issuable in
                   the aggregate under this plan and the Former Thermo
                   Environmental Corporation Incentive Stock Option
                   Plan is 1,450,195 shares, after adjustment to
                   reflect share increase approved in 1987; 3-for-2
                   stock splits effected in July 1993 and April 1995;
                   and 5-for-4 stock splits effected in December 1995
                   and October 1997).

     10.24         Thermo Instrument Systems Inc. - ThermoSpectra
                   Corporation Nonqualified Stock Option Plan (filed as
                   Exhibit 10.51 to the Registrant's Annual Report on
                   Form 10-K for the fiscal year ended December 31,
                   1994 [File No. 1-9786] and incorporated herein by
                   reference).


                                       19PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     10.25         Thermo Instrument Systems Inc. - ThermoQuest
                   Corporation Nonqualified Stock Option Plan (filed as
                   Exhibit 10.65 to Thermo Cardiosystems' Annual Report
                   on Form 10-K for the fiscal year ended December 30,
                   1995 [File No. 1-10114] and incorporated herein by
                   reference).

     10.26         Thermo Instrument Systems Inc. - Thermo BioAnalysis
                   Corporation Nonqualified Stock Option Plan (filed as
                   Exhibit 10.64 to Thermo Cardiosystems' Annual Report
                   on Form 10-K for the fiscal year ended December 30,
                   1995 [File No. 1-10114] and incorporated herein by
                   reference).

     10.27         Thermo Instrument Systems Inc. - Thermo Optek
                   Corporation Nonqualified Stock Option Plan (filed as
                   Exhibit 10.27 to the Registrant's Annual Report on
                   Form 10-K for the fiscal year ended December 28,
                   1996 [File No. 1-9786] and incorporated herein by
                   reference).

     10.28         Thermo Instrument Systems Inc. - Metrika Systems
                   Corporation Nonqualified Stock Option Plan.

     10.29         Thermo Instrument Systems Inc. - Thermo Vision
                   Corporation Nonqualified Stock Option Plan.

     10.30         Thermo Instrument Systems Inc. - ONIX Systems Inc.
                   Nonqualified Stock Option Plan.

                   In addition to the stock-based compensation plans of
                   the Registrant, the executive officers of the
                   Registrant may be granted awards under stock-based
                   compensation plans of Thermo Electron for services
                   rendered to the Registrant. The terms of such plans
                   are substantially the same as those of the
                   Registrant's Equity Incentive Plan.

     10.31         $210,000,000 Promissory Note dated as of March 27,
                   1997, issued by the Registrant to Thermo Electron
                   (filed as Exhibit 10.1 to the Registrant's Quarterly
                   Report on Form 10-Q for the quarter ended March 29,
                   1997, and incorporated herein by reference).

     10.32         $115,000,000 Promissory Note dated as of June 24,
                   1997, issued by the Registrant to Thermo Electron
                   (filed as Exhibit 10.1 to the Registrant's Quarterly
                   Report on Form 10-Q for the quarter ended June 28,
                   1997, and incorporated herein by reference).


                                       20PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     10.33         $3,800,000 Promissory Note dated as of July 14,
                   1997, issued by Thermo Vision to Thermo Electron
                   (filed as Exhibit 10 to Thermo Optek's Quarterly
                   Report on Form 10-Q for the quarter ended September
                   27, 1997, and incorporated herein by reference).

     10.34         $45,000,000 Promissory Note dated as of September
                   12, 1997, issued by ThermoSpectra to Thermo Electron
                   (filed as Exhibit 10 to ThermoSpectra's Quarterly
                   Report on Form 10-Q for the quarter ended September
                   27, 1997, and incorporated herein by reference).

     10.35         Amended and Restated Master Guarantee Reimbursement
                   and Loan Agreement dated as of December 5, 1997,
                   between Thermo Optek and Thermo Electron.

     10.36         Amended and Restated Master Guarantee Reimbursement and
                   Loan Agreement dated as of December 3, 1997, between
                   ThermoQuest and Thermo Electron.

     10.37         Amended and Restated Master Guarantee Reimbursement and
                   Loan Agreement dated as of December 3, 1997, between
                   Metrika Systems and Thermo Electron.

     10.38         Master Guarantee Reimbursement and Loan Agreement dated as
                   of November 14, 1997, between Thermo Vision and Thermo
                   Electron.

     10.39         Amended and Restated Master Guarantee Reimbursement and
                   Loan Agreement dated as of December 2, 1997, between
                   Thermo BioAnalysis and Thermo Electron.

     10.40         Amended and Restated Master Guarantee Reimbursement and
                   Loan Agreement dated as of December 4, 1997, between
                   ThermoSpectra and Thermo Electron.

     10.41         Amended and Restated Master Guarantee Reimbursement
                   and Loan Agreement dated as of January 5, 1998,
                   between ONIX Systems and Thermo Electron (filed as
                   Exhibit 10.5 to the Registration Statement of ONIX
                   Systems on Form S-1 [Reg. No. 333-45333] and
                   incorporated herein by reference).

     13            Annual Report to Shareholders for the year ended
                   January 3, 1998 (only those portions incorporated
                   herein by reference).

     21            Subsidiaries of the Registrant.

     23            Consent of Arthur Andersen LLP.

                                       21PAGE
<PAGE>
                                  EXHIBIT INDEX

    Exhibit
    Number         Description of Exhibit
    ------------------------------------------------------------------------
     27.1          Financial Data Schedule for the year ended January 3,
                   1998.

     27.2          Financial Data Schedule for the year ended December 28,
                   1996 (restated for the adoption of SFAS No. 128).

     27.3          Financial Data Schedule for the quarter ended March 29,
                   1997 (restated for the adoption of SFAS No. 128).

     27.4          Financial Data Schedule for the quarter ended June 28,
                   1997 (restated for the adoption of SFAS No. 128).

     27.5          Financial Data Schedule for the quarter ended
                   September 27, 1997 (restated for the adoption of
                   SFAS No. 128).