SECURITIES AND EXCHANGE COMMISSION
                              WASHINGTON, DC 20549
            ----------------------------------------------------

                AMENDMENT NO. 1 ON FORM 10-K/A TO 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 1-9567

                                 THERMEDICS INC.
             (Exact name of Registrant as specified in its charter)

Massachusetts                                                        04-2788806
(State or other jurisdiction of             (I.R.S. Employer Identification No.)
incorporation or organization)

470 Wildwood Street, P.O. Box 2999
Woburn, Massachusetts                                               01888-1799
(Address of principal executive offices)                            (Zip Code)

     Registrant's telephone number, including area code: (781) 622-1000

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

     Title of each class             Name of 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 $234,966,000.

As of January 30, 1998, the Registrant had 36,725,953 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>


                                                                     FORM 10-K/A

                                     PART IV

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

(c) Exhibits

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

    23 Consent of Arthur Andersen LLP.

<PAGE>


                                                                     FORM 10-K/A

                                 THERMEDICS INC.

                                   SIGNATURES


    Pursuant to the requirements of Section 13 or 15(d) of the Securities
Exchange Act of 1934, the Registrant has duly caused this Amendment No. 1 on
Form 10-K/A to be signed on its behalf by the undersigned, thereunto duly
authorized.

Date:  February 16, 1999

                                          THERMEDICS INC.



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