Exhibit-10.1

                                   REPLACEMENT
                                 PROMISSORY NOTE


US $2,100,000                                                    30 March 2001
                                                                 Columbia, MD


           THIS INSTRUMENT AND ALL RIGHTS OF THE PARTIES HEREUNDER ARE
            SUBJECT TO AND GOVERNED BY THE TERMS AND CONDITIONS OF A
            SUBORDINATION AND INTERCREDITOR AGREEMENT DATED MARCH 30,
         2001, BY AND BETWEEN MANTECH INTERNATIONAL CORPORATION AND
          NATIONAL BANK OF CANADA. WITHOUT LIMITATION OF THE FOREGOING,
          ALL RIGHTS OF PAYMENT, LIEN RIGHTS, AND ENFORCEMENT RIGHTS OF
          THE HOLDER OF THIS INSTRUMENT, ARE EXPRESSLY SUBORDINATED AND
          SUBJECT TO THE RIGHTS OF NATIONAL BANK OF CANADA, AS PROVIDED
                IN THE SUBORDINATION AND INTERCREDITOR AGREEMENT.


                                R E C I T A L S:
                                - - - - - - - -

     A. Reference is made to that certain Secured  Promissory Note dated October
2, 2000 in the original  principal  amount of One Million Eight Hundred Thousand
Dollars (US $1,800,000), made by GSE Systems, Inc., a Delaware corporation, (the
"Borrower")  payable to the order of ManTech  International  Corporation,  a New
Jersey  corporation,  and its successors and assigns (the "Lender"),  as amended
and modified pursuant to that certain Allonge and First  Modification to Secured
Promissory Note dated as of January 25, 2001, increasing the principal amount of
the Secured  Promissory  Note to Two Million  One Hundred  Thousand  Dollars (US
$2,100,000).  The  Secured  Promissory  Note,  as so amended  and  modified,  is
referred to herein as the "Original Note". The aggregate Two Million One Hundred
Thousand  Dollars (US  $2,100,000)  of loans  evidenced by the Original  Note is
referred to herein as the "Loan."

     B. Pursuant to the Original  Note,  the Loan was secured by the  Borrower's
equity  interest  in Avantium  International  B.V.  (formerly  known as Avantium
B.V.),  a private  company with limited  liability  formed under the laws of The
Netherlands   ("Avantium")  (the  "Collateral"),   which  security  interest  is
subordinate  to the  security  interest  held by  National  Bank of Canada,  the
Borrower's principal lender ("NBOC"), in the Collateral.

     C. In connection with various proposed new credit  accommodations from NBOC
to the Borrower,  NBOC has prohibited any security for the Loan and the Original
Note.

     D. The Lender has agreed to release its security interest in any collateral
securing the Loan or the Note  (including,  but not limited to, the Collateral),
in order to comply with NBOC's requirement.


     NOW,  THEREFORE,  for Ten  Dollars  ($10.00)  and other  good and  valuable
consideration, the receipt and sufficiency of which are hereby acknowledged, the
Borrower  and the  Lender  agree  that this Note is hereby  substituted  for and
replaces the Original Note.

                                    * * * * *
     FOR VALUE RECEIVED the undersigned Borrower unconditionally promises to pay
to the order of the Lender,  at the  account  specified  in writing,  or at such
place in the United  States of America as a holder  hereof may from time to time
direct in  writing,  in lawful  money of the  United  States of  America  and in
immediately  available  funds,  the principal  amount of TWO MILLION ONE HUNDRED
THOUSAND DOLLARS (US $2,100,000),  together with all accrued but unpaid interest
thereon (the  "Note").  The interest rate shall be computed on the actual number
of days elapsed  (based on a 360-day year of twelve equal  months) on the unpaid
balance of such principal amount at the rate described below.

     Interest shall be at the fluctuating prime rate of interest established and
declared by Mellon Bank,  N.A. (the "Prime Rate") plus one (1) percentage  point
per annum,  to be paid in lawful money of the United  States and in  immediately
available funds.  Interest shall be compounded monthly,  based on the Prime Rate
in effect on the last day of such month.

     The Borrower may borrow under this Note subject to the principal  amount of
the Note.  The  Borrower may prepay this Note in whole or in part at any time or
from time to time without penalty or premium. Any such prepayment shall first be
applied to accrued and unpaid interest,  if any, and thereafter to the principal
outstanding under this Note.

     The Borrower shall repay this Note as follows:

     1.   Payments  of accrued  interest  only  shall be due and  payable on the
          first day of each month,  commencing on July 1, 2001,  and  continuing
          through the Maturity Date (as defined below).

     2.   Twenty-four  (24) equal  installments  of  principal  shall be due and
          payable on the first day of each month,  commencing  on April 1, 2004,
          and continuing through the Maturity Date.

     3.   The entire principal  balance of this Note,  together with all accrued
          and unpaid interest thereon and other sums payable  hereunder,  unless
          sooner  accelerated  pursuant to the terms of this Note,  shall be due
          and payable in full on April 1, 2006 (the "Maturity Date").

     This Note may be convertible,  at the Lender's option, in whole or in part,
into shares of  convertible  preferred  stock of the Borrower at the rate of One
Hundred Dollars ($100) per share. Such preferred stock is to be convertible into
common stock of the Borrower.  The other rights and preferences of the preferred
stock  shall  be  mutually  agreed  upon by the  Borrower  and the  Lender.  The
authorization  and/or  issuance of such  preferred  stock,  and the common stock
issuable upon exercise of the conversion  right of such preferred  stock, may be
subject to the approval of the Borrower's stockholders, if it is determined that
such stockholder approval is required.

     The Lender hereby releases, discharges and terminates its security interest
in any and all collateral security securing the Loan or the Note (including, but
not limited to, the Collateral), and releases to the Borrower any and all right,
title and interest which the Lender may have acquired in and to such  collateral
(including,  but not limited to, the Collateral),  pursuant to the Original Note
or otherwise. The Lender agrees to execute such documents, and to take all other
actions reasonably  requested by the Borrower,  to evidence the Lender's release
of its security interest in all of such collateral,  including,  but not limited
to, the Collateral.

     Upon  execution of this Note by the  Borrower,  the Lender shall return the
Original Note to the Borrower, marked "Cancelled."

     The Borrower  represents and warrants to the Lender (which  representations
and  warranties  will be deemed to be  repeated  by the  Borrower on each day on
which the Note or other obligation of the Borrower remains outstanding) that:

(A)  It is a corporation  duly organized,  validly existing and in good standing
     under the laws of the State of Delaware;

(B)  It is duly  qualified,  in good  standing and  authorized to do business in
     each  jurisdiction  where  because  of  the  nature  of its  activities  or
     properties such qualification is required by applicable laws or the failure
     to be so qualified could have a material adverse effect on the Borrower;

(C)  It has the  power to enter  into the  Note,  to  deliver  this  Note and to
     perform its obligations  under this Note and has taken all necessary action
     to authorize such execution, delivery and performance;

(D)  The execution,  delivery and  performance of this Note by the Borrower does
     not and will not violate or  conflict  with any law  applicable  to it, any
     provision  of its  organizational  documents,  any order or judgment of any
     court or other agency of  government  applicable to it or any of its assets
     or any  contractual  restriction  binding on or  affecting it or any of its
     assets;

(E)  All governmental and other consents,  authorizations,  approvals,  licenses
     and orders that are  required to have been  obtained by it with  respect to
     this Note and the transactions  contemplated  herein have been obtained and
     are in full  force and  effect  and all  conditions  of any such  consents,
     authorizations, approvals, licenses and orders have been complied with;

(F)  Its  obligations  under this Note  constitute its legal,  valid and binding
     obligations, enforceable in accordance with their respective terms (subject
     to applicable bankruptcy, reorganization, insolvency, moratorium or similar
     laws   affecting   creditors'   rights   generally   and  subject,   as  to
     enforceability,  to equitable principles of general application (regardless
     of whether enforcement is sought in a proceeding in equity or at law)); and

(G)  There  is not  pending  or  threatened  against  it  any  action,  suit  or
     proceeding at law or in equity or before any court, tribunal,  governmental
     body, agency or official or any arbitrator that is likely to (1) affect the
     legality, validity or enforceability against it of this Note or its ability
     to  perform  its  obligations  hereunder,  or  (2)  materially  affect  its
     operations, business, property or assets or financial or other condition.

     The Borrower  covenants to the Lender that,  so long as the Borrower has or
may have any obligation under this Note:

(a)  Except  as  expressly  set forth  herein,  this Note  shall  constitute  an
     unsecured  direct,  general and  unconditional  obligation of the Borrower.
     Notwithstanding  anything  to the  contrary  stated  herein,  this  Note is
     subordinate   in  payment,   priority  and  collection  to  the  Borrower's
     obligations to NBOC, the terms of which subordination are set forth in that
     certain  Subordination  and  Intercreditor  Agreement  dated as of the date
     hereof by and between the Lender and NBOC.

(b)  It will  maintain  in full force and effect and comply  with all  consents,
     authorizations, approvals, licenses and orders of any governmental or other
     authority  that are required to be obtained by it with respect to this Note
     or the transactions contemplated hereby and will use all reasonable efforts
     to obtain any such consents, authorizations, approvals, licenses and orders
     that may become necessary in the future;

(c)  It will comply in all material respects with all applicable laws and orders
     to which it may be subject, if failure so to comply would materially impair
     its ability to perform its obligations under this Note; and

(d)  It will  notify the Lender upon the  occurrence  of an Event of Default (as
     defined below).

     The  occurrence  of  any  of  the  following  events  or  conditions  shall
constitute an event of default (each, an "Event of Default") with respect to the
Borrower under this Note:

(I)  Any amounts due under this Note are not paid (i) with  respect to principal
     and/or  interest  within Ten (10) calendar days after the date when due, or
     (ii) with respect to other  amounts  within Twenty (20) calendar days after
     the due date thereof;

(II) Except as  otherwise  provided  in  subsection  (I) above,  the  failure or
     refusal of the  Borrower  to properly  perform,  observe or comply with any
     condition,  obligation, or covenant or agreement to be performed,  observed
     or complied with by the Borrower in this Note,  and such failure or refusal
     continues for a period of Ten (10) calendar days, or for such lesser period
     as stipulated in this Note after written notice thereof from the Lender;

(III)A  representation  made or repeated or deemed to have been made or repeated
     by the Borrower in this Note proves to have been  incorrect  or  misleading
     when made or  repeated  or deemed to have been made or  repeated;  provided
     that in the case of a repeated  representation,  the Borrower  shall have a
     period of Ten (10)  calendar days after notice from the Lender to cure such
     incorrect or misleading representation; or

(IV) Borrower  (1) is  dissolved  (other  than  pursuant to a  consolidation  or
     merger);  (2) becomes  insolvent  or is unable to pay its debts or fails or
     admits in writing its  inability  generally to pay its debts as they become
     due; (3) makes a general assignment, arrangement or composition with or for
     the benefit of its creditors; (4) institutes or has instituted against it a
     proceeding  seeking a judgment of  insolvency  or  bankruptcy  or any other
     relief  under  any  bankruptcy  or  insolvency  law or  other  similar  law
     affecting  creditors' rights, or a petition is presented for its winding-up
     or  liquidation  which is not dismissed,  discharged,  stayed or restrained
     within  Thirty  (30)  calendar  days  of the  institution  or  presentation
     thereof;  (5) has a resolution  passed for its  winding-up  or  liquidation
     (other than pursuant to a  consolidation  or merger);  (6) seeks or becomes
     subject to the  appointment of an  administrator,  provisional  liquidator,
     conservator,  receiver, trustee, custodian or other similar official for it
     or for all or substantially all its assets; (7) has a distress,  execution,
     attachment,  sequestration or other legal process levied,  enforced or sued
     on or against all or  substantially  all its assets and such process is not
     dismissed,  discharged,  stayed or  restrained,  in each case within Thirty
     (30) calendar days  thereafter;  (8) causes or is subject to any event with
     respect to it which, under the applicable laws of any jurisdiction,  has an
     analogous  effect to any of the  events  specified  in  clauses  (1) to (7)
     (inclusive);  or (9)  takes any  corporate  action  in  furtherance  of, or
     otherwise indicates its written consent to, written approval of, or written
     acquiescence in, any of the foregoing acts.

(V)  The Borrower is a party to any merger or  consolidation or transfers all or
     substantially  all its assets to another person or entity without the prior
     written consent of the Lender and,  pursuant to such merger,  consolidation
     or transfer the creditworthiness of the resulting,  surviving or transferee
     entity  is  weaker  than  that of the  Borrower  immediately  prior to such
     action.

     Upon the occurrence and during the continuance of an Event of Default,  the
Lender will have the option,  upon notice to the  Borrower,  of declaring any or
all unpaid amounts under the Note, together with unpaid accrued interest thereon
to be immediately due and payable.

     All payments and repayments  shall be made on a day that banks are open for
business in the  Commonwealth of Virginia (a "Business  Day"). If any payment in
respect  of this Note  becomes  due on a day that is not a  Business  Day,  such
payment shall be made on the next succeeding Business Day, and such extension of
the time  shall be  included  in  computing  interest  in  connection  with such
payment.

     All payments to the Lender  hereunder shall be made by wire transfer to the
account specified by the Lender in writing from time to time, in lawful money of
the United States of America.

     All payments  made by the Borrower in respect of principal of, and interest
on the Note and all other amounts  payable in respect of this Note or otherwise,
will be made without  set-off,  counterclaim  or other  defense and will be made
free and clear of, and without  deduction  or  withholding  for,  any present or
future taxes, levies,  imposts,  duties,  fees,  assessments or other charges of
whatever nature now or hereafter imposed by any jurisdiction or by any political
subdivision or taxing authority  thereof or therein and all interest,  penalties
or similar  liabilities with respect thereto  (collectively,  the "Taxes").  The
Borrower shall pay on demand all stamp, documentary and other similar duties and
taxes,  if any,  to which  this Note from time to time may be subject to or give
rise.  If the Borrower is required by  applicable  law to make any  deduction or
withholding on any payment as described  above in respect of Taxes or otherwise,
the Borrower shall: (i) promptly notify the Lender of such occurrence;  (ii) pay
to the relevant  taxation or other  authorities the full amount of the deduction
or  withholding  within the time  allowed;  (iii)  furnish to the Lender  within
Thirty  (30)  calendar  days of such  payment,  an  official  receipt  from such
authorities for all amounts so deducted or withheld;  and (iv) pay to the Lender
an additional amount so that the Lender receives on the due date of such payment
the full  amount  the  Lender  would  have  received  had no such  deduction  or
withholding taken place.

     The  Borrower  agrees  to pay on  demand  all of  the  Lender's  costs  and
expenses,   including,  without  limitation,   reasonable  attorneys'  fees  and
expenses, in connection with any judgment relating to the collection of any sums
due to the Lender and the  enforcement  or protection of its rights or interests
hereunder, whether suit be brought or not.

     THIS NOTE SHALL BE GOVERNED BY AND CONSTRUED IN ACCORDANCE WITH THE LAWS OF
THE COMMONWEALTH OR VIRGINIA, WITHOUT REGARD TO ITS CONFLICTS OF LAW RULES.

     Nothing  contained  herein shall limit or impair the right of the Lender to
institute any suit, action, motion or proceeding in any other court of competent
jurisdiction,  nor shall the taking of any suit,  action or proceeding in one or
more jurisdictions preclude the taking of proceedings in any other jurisdiction,
whether concurrently or not.

     The  Borrower  may not  assign  any of its  rights or  delegate  any of its
obligations  under this Note (or any part  thereof)  without  the prior  written
consent  of the  Lender,  which  consent  shall  not be  unreasonably  withheld;
provided,  however, nothing contained herein shall limit the right of the Lender
to assign any of its rights or obligations hereunder.

     The Borrower hereby waives diligence,  presentment,  protest,  demand,  and
notice of every kind and,  to the full  extent  permitted  by law,  the right to
plead any  statute  of  limitations  as a defense to any  demand  hereunder  and
further agrees to be bound hereby,  notwithstanding any extension,  modification
or waiver by any  holder of this Note or upon the  discharge  or  release of any
obligor hereunder.

     If any clause,  provision  or section of this Note shall be held illegal or
invalid by any court, the illegality or invalidity of such clause,  provision or
section  shall not affect the  remainder of this Note,  which shall be construed
and enforced as if such illegal or invalid clause,  provision or section had not
been  contained in this Note. If any  agreement or obligation  contained in this
Note is held to be in  violation  of  applicable  law,  then such  agreement  or
obligation  shall be deemed to be the  agreement or  obligation  of the Borrower
only to the extent permitted by applicable law.


         IN WITNESS WHEREOF, the undersigned have caused this Note to be
executed as of the date first above written.

BORROWER:

GSE Systems, Inc.



By:      ______________________
Name:    Jeffery G. Hough
Title:   Senior Vice President


LENDER:

ManTech International Corporation



By:      ______________________
Name:
Title: