<PAGE>
                                                                Exhibit No. 4(c)

                             SUB-ADVISORY CONTRACT

          Agreement made as of March 1, 2000 ("Contract") between MITCHELL
HUTCHINS ASSET MANAGEMENT INC., a Delaware corporation ("Mitchell Hutchins"),
and DSI INTERNATIONAL MANAGEMENT, INC., a Delaware corporation ("Sub-Adviser").


                                   RECITALS
                                   --------

          (1) Mitchell Hutchins has entered into an Investment Advisory and
Administration Agreement, dated March 1, 2000 ("Management Agreement"), with
Mitchell Hutchins Securities Trust ("Trust"), an open-end management investment
company registered under the Investment Company Act of 1940, as amended ("1940
Act");

          (2) The Trust offers for public sale two distinct series of shares of
beneficial interest which correspond to distinct portfolios one of which is
known as PaineWebber Enhanced Nasdaq-100 Fund ("Fund") and may offer additional
district series in the future;

          (3) Under the Management Agreement, Mitchell Hutchins has agreed to
provide certain investment advisory and administrative services to the Fund;

          (4) The Management Agreement permits Mitchell Hutchins to delegate
certain of its duties as investment adviser thereunder to a sub-adviser;

          (5) Mitchell Hutchins desires to retain the Sub-Adviser to furnish
certain investment advisory services with respect to the PaineWebber Enhanced
Nasdaq-100 Fund; and

          (6) The Sub-Adviser is willing to furnish such services;

          NOW, THEREFORE, in consideration of the premises and mutual covenants
herein contained, Mitchell Hutchins and the Sub-Adviser agree as follows:

          1.  Appointment. Mitchell Hutchins hereby appoints the Sub-Adviser as
              -----------
an investment sub-adviser with respect to the Fund for the period and on the
terms set forth in this Contract. The Sub-Adviser accepts that appointment and
agrees to render the services herein set forth, for the compensation herein
provided.

          2.  Duties as Sub-Adviser.
              ----------------------

         (a) Subject to the supervision and direction of the Trust's Board of
Trustees ("Board") and review by Mitchell Hutchins, and any written guidelines
adopted by the Board or Mitchell Hutchins, the Sub-Adviser will provide a
continuous investment program for the Fund, including investment research and
management with respect to all securities and investments and cash equivalents
in the Fund.  The Sub-Adviser will determine from time to time what investments
will be purchased, retained or sold by the Fund.  The Sub-Adviser will be
responsible for placing
<PAGE>

purchase and sell orders for investments and for other related transactions. The
Sub-Adviser will provide services under this Contract in accordance with the
Fund's investment objective, policies and restrictions as stated in the Trust's
currently effective registration statement under the 1940 Act, and any
amendments or supplements thereto ("Registration Statement").

          (b) The Sub-Adviser agrees that, in placing orders with brokers, it
will obtain the best net result in terms of price and execution; provided that,
on behalf of the Fund, the Sub-Adviser may, in its discretion, use brokers who
provide the Sub-Adviser with research, analysis, advice and similar services to
execute portfolio transactions, and the Sub-Adviser may pay to those brokers in
return for brokerage and research services a higher commission than may be
charged by other brokers, subject to the Sub-Adviser's determining in good faith
that such commission is reasonable in terms either of the particular transaction
or of the overall responsibility of the Sub-Adviser to the Fund and its other
clients and that the total commissions paid by the Fund will be reasonable in
relation to the benefits to the Fund over the long term. In no instance will
portfolio securities be purchased from or sold to the Sub-Adviser, or any
affiliated person thereof, except in accordance with the federal securities laws
and the rules and regulations thereunder. The Sub-Adviser may aggregate sales
and purchase orders with respect to the assets of the Fund with similar orders
being made simultaneously for other accounts advised by the Sub-Adviser or its
affiliates. Whenever the Sub-Adviser simultaneously places orders to purchase or
sell the same security on behalf of the Fund and one or more other accounts
advised by the Sub-Adviser, the orders will be allocated as to price and amount
among all such accounts in a manner believed to be equitable over time to each
account. Mitchell Hutchins recognizes that in some cases this procedure may
adversely affect the results obtained for the Fund.

          (c) The Sub-Adviser will maintain all books and records required to be
maintained pursuant to the 1940 Act and the rules and regulations promulgated
thereunder with respect to actions by the Sub-Adviser on behalf of the Fund, and
will furnish the Board and Mitchell Hutchins with such periodic and special
reports as the Board or Mitchell Hutchins reasonably may request. In compliance
with the requirements of Rule 31a-3 under the 1940 Act, the Sub-Adviser hereby
agrees that all records that it maintains for the Fund are the property of the
Trust, agrees to preserve for the periods prescribed by Rule 31a-2 under the
1940 Act any records that it maintains for the Trust and that are required to be
maintained by Rule 31a-1 under the 1940 Act, and further agrees to surrender
promptly to the Trust any records that it maintains for the Fund upon request by
the Trust.

          (d) At such times as shall be reasonably requested by the Board or
Mitchell Hutchins, the Sub-Adviser will provide the Board and Mitchell Hutchins
with economic and investment analyses and reports as well as quarterly reports
setting forth the performance of the international segment of the Fund's
investments and make available to the Board and Mitchell Hutchins any economic,
statistical and investment services that the Sub-Adviser normally makes
available to its institutional or other customers.

          (e) In accordance with procedures adopted by the Board, as amended
from time to time, the Sub-Adviser is responsible for assisting in the fair
valuation of all portfolio securities and will use its reasonable efforts to
arrange for the provision of a price(s) from a party(ies) independent of
<PAGE>

the Sub-Adviser for each portfolio security for which the custodian does not
obtain prices in the ordinary course of business from an automated pricing
service.

          3.   Further Duties. In all matters relating to the performance of
               --------------
this Contract, the Sub-Adviser will act in conformity with the Trust's Trust
Instrument, By-Laws and Registration Statement and with the written instructions
and written directions of the Board and Mitchell Hutchins; and will comply with
the requirements of the 1940 Act and the Investment Advisers Act of 1940, as
amended ("Advisers Act") and the rules under each, Subchapter M of the Internal
Revenue Code ("Code"), as applicable to regulated investment companies; the
diversification requirements applicable to the Fund under Section 817(h) of the
Code; and all other federal and state laws and regulations applicable to the
Trust and the Fund. Mitchell Hutchins agrees to provide to the Sub-Adviser
copies of the Trust's Trust Instrument, By-Laws, Registration Statement, written
instructions and directions of the Board and Mitchell Hutchins, and any
amendments or supplements to any of these materials as soon as practicable after
such materials become available; and further agrees to identify to the Sub-
Adviser in writing any broker-dealers that are affiliated with Mitchell Hutchins
(other than PaineWebber Incorporated and Mitchell Hutchins itself).

          4.   Expenses. During the term of this Contract, the Sub-Adviser will
               --------
bear all expenses incurred by it in connection with its services under this
Contract.

          5.   Compensation.
               -------------

          (a) For the services provided and the expenses assumed by the Sub-
Adviser pursuant to this Contract, Mitchell Hutchins, not the Fund, will pay to
the Sub-Adviser a sub-advisory fee, computed daily and paid monthly, at an
annual rate of 0.35% of the Fund's average daily net assets.

          (b) The fee shall be accrued daily and payable monthly to the Sub-
Adviser on or before the last business day of the next succeeding calendar
month.

          (c) If this Contract becomes effective or terminates before the end of
any month, the fee for the period from the effective date to the end of the
month or from the beginning of such month to the date of termination, as the
case may be, shall be pro-rated according to the proportion that such period
bears to the full month in which such effectiveness or termination occurs.

          6.  Limitation of Liability. The Sub-Adviser shall not be liable for
              -----------------------
any error of judgment or mistake of law or for any loss suffered by the Fund,
the Trust, its shareholders or by Mitchell Hutchins in connection with the
matters to which this Contract relates, except a loss resulting from willful
misfeasance, bad faith or gross negligence on its part in the performance of its
duties or from reckless disregard by it of its obligations and duties under this
Contract. Nothing in this paragraph shall be deemed a limitation or waiver of
any obligation or duty that may not by law be limited or waived.

          7.  Representations of Sub-Adviser. The Sub-Adviser represents,
              ------------------------------
warrants and agrees as follows:
<PAGE>

          (a) The Sub-Adviser (i) is registered as an investment adviser under
the Advisers Act and will continue to be so registered for so long as this
Contract remains in effect; (ii) is not prohibited by the 1940 Act or the
Advisers Act from performing the services contemplated by this Contract; (iii)
has met and will seek to continue to meet for so long as this Contract remains
in effect, any other applicable federal or state requirements, or the applicable
requirements of any regulatory or industry self-regulatory agency necessary to
be met in order to perform the services contemplated by this Contract; (iv) has
the authority to enter into and perform the services contemplated by this
Contract; and (v) will promptly notify Mitchell Hutchins of the occurrence of
any event that would disqualify the Sub-Adviser from serving as an investment
adviser of an investment company pursuant to Section 9(a) of the 1940 Act or
otherwise.

          (b) The Sub-Adviser has adopted a written code of ethics complying
with the requirements of Rule 17j-1 under the 1940 Act and will provide Mitchell
Hutchins and the Board with a copy of such code of ethics, together with
evidence of its adoption. Within forty-five days of the end of the last calendar
quarter of each year that this Contract is in effect, the president or a vice-
president of the Sub-Adviser shall certify to Mitchell Hutchins that the Sub-
Adviser has complied with the requirements of Rule 17j-1 during the previous
year and that there has been no violation of the Sub-Adviser's code of ethics
or, if such a violation has occurred, that appropriate action was taken in
response to such violation. Upon the written request of Mitchell Hutchins, the
Sub-Adviser shall permit Mitchell Hutchins, its employees or its agents to
examine the reports required to be made to the Sub-Adviser by Rule 17j-1(c)(1)
and all other records relevant to the Sub-Adviser's code of ethics.

          (c) The Sub-Adviser has provided Mitchell Hutchins with a copy of its
Form ADV, which as of the date of this Agreement is its Form ADV as most
recently filed with the Securities and Exchange Commission ("SEC") and promptly
will furnish a copy of all amendments to Mitchell Hutchins at least annually.

          (d) The Sub-Adviser will notify Mitchell Hutchins of any change of
control of the Sub-Adviser, including any change of its general partners or 25%
shareholders, as applicable, and any changes in the key personnel who are either
the portfolio manager(s) of the Fund or senior management of the Sub-Adviser, in
each case prior to, or promptly after, such change.


          (e) The Sub-Adviser agrees that neither it, nor any of its affiliates,
will in any way refer directly or indirectly to its relationship with the Trust,
the Fund, Mitchell Hutchins or any of their respective affiliates in offering,
marketing or other promotional materials without the prior express written
consent of Mitchell Hutchins.

          (8) Services Not Exclusive.  The services furnished by the Sub-Adviser
              ----------------------
hereunder are not to be deemed exclusive and the Sub-Adviser shall be free to
furnish similar services to others so long as its services under this Contract
are not impaired thereby or unless otherwise agreed to by the parties hereunder
in writing.  Nothing in this Contract shall limit or restrict the right of any
director, officer or employee of the Sub-Adviser, who may also be a trustee,
officer or employee of the Trust, to engage in any other business or to devote
his or her time and attention in part to the management or other aspects of any
other business, whether of a similar nature or a dissimilar nature.
<PAGE>

          9.  Duration and Termination.
              -------------------------

          (a) This Contract shall become effective upon the date first above
written, provided that this Contract shall not take effect unless it has first
been approved: (i) by a vote of a majority of those trustees of the Trust who
are not parties to this Contract or interested persons of any such party, cast
in person at a meeting called for the purpose of voting on such approval, and
(ii) by vote of a majority of the Fund's outstanding securities.

          (b) Unless sooner terminated as provided herein, this Contract shall
continue in effect for two years from its effective date. Thereafter, if not
terminated, this Contract shall continue automatically for successive periods of
twelve months each, provided that such continuance is specifically approved at
least annually: (i) by a vote of a majority of those trustees of the Trust who
are not parties to this Contract or interested persons of any such party, cast
in person at a meeting called for the purpose of voting on such approval, and
(ii) by the Board or by vote of a majority of the outstanding voting securities
of the Fund.

          (c) Notwithstanding the foregoing, this Contract may be terminated at
any time, without the payment of any penalty, by vote of the Board or by a vote
of a majority of the outstanding voting securities of the Fund on 60 days'
written notice to the Sub-Adviser. This Contract may also be terminated, without
the payment of any penalty, by Mitchell Hutchins: (i) upon 120 days' written
notice to the Sub-Adviser; (ii) upon material breach by the Sub-Adviser of any
representations and warranties set forth in Paragraph 7 of this Contract, if
such breach has not been cured within a 20 day period after notice of such
breach; or (iii) immediately if, in the reasonable judgment of Mitchell
Hutchins, the Sub-Adviser becomes unable to discharge its duties and obligations
under this Contract, including circumstances such as financial insolvency of the
Sub-Adviser or other circumstances that could adversely affect the Fund. The
Sub-Adviser may terminate this Contract at any time, without the payment of any
penalty, on 120 days written notice to Mitchell Hutchins. This Contract will
terminate automatically in the event of its assignment or upon termination of
the Advisory Contract as it relates to the Fund.

          10. Amendment of this Contract. No provision of this Contract may be
              ---------------------------
changed, waived, discharged or terminated orally, but only by an instrument in
writing signed by the party against whom enforcement of the change, waiver,
discharge or termination is sought. No amendment of this Contract shall be
effective until approved by a vote of a majority of those trustees of the Trust
who are not parties to this Contract or interested persons of any such party.

          11. Governing Law. This Contract shall be construed in accordance with
              -------------
the 1940 Act and the laws of the State of Delaware, without giving effect to the
conflicts of laws principles thereof. To the extent that the applicable laws of
the State of Delaware conflict with the applicable provisions of the 1940 Act,
the latter shall control.

          12. Miscellaneous.  The captions in this Contract are included for
              --------------
convenience of reference only and in no way define or delimit any of the
provisions hereof or otherwise affect their construction or effect.  If any
provision of this Contract shall be held or made invalid by a court decision,
statute, rule or otherwise, the remainder of this Contract shall not be affected
thereby.  This Contract shall be binding upon and shall inure to the benefit of
the parties hereto and their
<PAGE>

respective successors. As used in this Contract, the terms "majority of the
outstanding voting securities," "affiliated person," "interested person,"
"assignment," "broker," "investment adviser," "net assets," "sale," "sell" and
"security" shall have the same meaning as such terms have in the 1940 Act,
subject to such exemption as may be granted by the SEC by any rule, regulation
or order. Where the effect of a requirement of the federal securities laws
reflected in any provision of this Contract is made less restrictive by a rule,
regulation or order of the SEC, whether of special or general application, such
provision shall be deemed to incorporate the effect of such rule, regulation or
order. This Contract may be signed in counterpart.

          13. Notices. Any notice herein required is to be in writing and is
              -------
deemed to have been given to the Sub-Adviser or Mitchell Hutchins upon receipt
of the same at their respective addresses set forth below. All written notices
required or permitted to be given under this Contract will be delivered by
personal service, by postage mail - return receipt requested or by facsimile
machine or a similar means of same day delivery which provides evidence of
receipt (with a confirming copy by mail as set forth herein). All notices
provided to Mitchell Hutchins will be sent to the attention of Victoria E.
Schonfeld, General Counsel. All notices provided to the Sub-Adviser will be sent
to the attention of [NAME] [TITLE].

                    [rest of page left intentionally blank]
<PAGE>

     IN WITNESS WHEREOF, the parties hereto have caused this instrument to be
executed by their duly authorized signatories as of the date and year first
above written.


                                            MITCHELL HUTCHINS ASSET
                                             MANAGEMENT INC.
                                            51 West 52nd Street
                                            New York, New York  10019-6114
Attest:

By: /s/ Cristina Paradiso                    By: /s/ Dianne E. O'Donnell
    ________________________________             ____________________________
    Name: Cristina Paradiso                      Name: Dianne E. O'Donnell
    Title: Vice President                        Title: Senior Vice President



                                            DSI INTERNATIONAL
                                             MANAGEMENT, INC.
                                            301 Merritt 7
                                            Norwalk, CT  06851

Attest:

By: /s/ Tina Charno                         By: /s/ John J. Holmgren, Jr.
    _____________________________               _______________________________
    Name: Tina Charno                           Name: John J. Holmgren, Jr.
    Title: Portfolio Accounting                 Title: Executive Vice President