EXHIBIT 10.6

                                PROMISSORY NOTE

$500,000.00 (U.S.)                                          September 8, 1998

      FOR VALUE RECEIVED, and intending to be legally bound, the undersigned
John G. Vondrak, a Texas resident having his home address at 2111 Welch, A 216,
Houston, TX 77019 (Referred to herein as "Maker"), promises to pay to the order
of APPLE ORTHODONTIX, INC., a Delaware corporation whose corporate headquarters
are located at 2777 Allen Parkway, Suite 700, Houston, Texas, or its assignee(s)
or transferee(s) ("Payee") in lawful money of the United States of America, on
such terms and at such times as are set forth herein, the principal sum of FIVE
HUNDRED THOUSAND AND NO/100 DOLLARS ($500,000.00), or so much thereof as may be
outstanding hereunder prior to maturity, together with interest at the annual
rates specified below, on any and all principal amounts remaining unpaid
hereunder and from time to time outstanding from the date hereof until such
principal amounts are paid.

      The unpaid principal amount of this Note shall bear interest from the date
hereof until paid at a rate per annum equal to the lesser of (i) eight and
one-half percent (8.50%) , or (ii) the Highest Lawful Rate (as hereinafter
defined).

      Principal and interest shall be due and payable in full on September 9,
2000, unless the maturity of this Note is accelerated to an earlier date.

      The Maker reserves the right to prepay this Note, in whole or in part, at
any time, without penalty or notice. All prepayments hereunder, whether
designated as payments of principal or interest, shall be applied first to
accrued and unpaid interest, if any, and then to principal.

      If any of the following shall occur, then the holder of this Note may, at
its option, declare all of the indebtedness evidenced by this Note to be
forthwith due and payable and, in addition, exercise all rights and remedies
otherwise permitted under applicable law: any payment, whether of principal,
interest or other amounts, provided for hereunder shall not be paid when due,
and such default in payment shall continue for a period of fifteen days
following receipt by Maker of written notice from Payee of non-payment; or shall
admit their inability to pay his debts as they mature, or shall make an
assignment for the benefit of its creditors; or proceedings in bankruptcy, or
for reorganization of Maker, or for the readjustment of any of their debts under
the United States Bankruptcy Code, as amended, or any proceeding under any other
laws, whether state or federal, for the relief of debtors, now or hereafter
existing, shall be commenced against or by Maker; or a receiver or trustee shall
be appointed for Maker, or for any substantial part of their assets.

      All signers and endorsers of this Note are to be regarded as principals as
to their respective joint and several liability to any holder hereof, and Maker
and each of the guarantors, sureties and endorsers hereof, to the fullest extent
permitted by applicable law, hereby expressly and severally waive grace, demand,
presentment for payment, notice of dishonor, notice of nonpayment, protest and
notice of protest, notice of intent to accelerate, notice of acceleration of the
indebtedness due hereunder, any other notice of any kind with respect to this
Note, and

                                 Page 1 of 3
<PAGE>
diligence in collecting this Note or enforcing any security rights of any holder
hereof under any document securing this Note, and agree (i) that any holder of
this Note may, at any time, and from time to time, extend the date of maturity
of all or any part hereof or renew this Note, or accept partial payments
hereunder, or rearrange the indebtedness evidenced hereby, in each case without
notifying or obtaining the consent of Maker or any such other principal hereof,
each of whom shall remain fully obligated for the payment hereof; (ii) that it
will not be necessary for any holder hereof, in order to enforce payment of this
Note, first to institute or exhaust its remedies against Maker or any other
party liable therefor or to enforce its rights against any security for this
Note; and (iii) to any substitution, exchange or release of any security
hereafter given for this Note, or the release of any party primarily or
secondarily liable hereon, or any other circumstance which might otherwise
constitute a defense available to, or a discharge of, Maker or any guarantor,
surety or endorser with respect to its obligations in respect of this Note.

      As of the date hereof, this Note is secured by a security interest in
250,000 shares of Class A or B Common Stock of Apple Orthodontix, Inc. pursuant
to a stock pledge agreement.

      To the fullest extent permitted by applicable law, if this Note is
collected by suit or legal proceedings or through the probate court or
bankruptcy proceedings, Maker agrees to pay to the holder hereof the reasonable
costs and reasonable attorney's fees incurred in the collection hereof.

      No modification or waiver of any of the provisions of this Note shall be
effective unless in writing, signed by the holder hereof, and only to the extent
set forth therein; and no waiver shall be effective except in the specific
instance for which given; and no failure by the holder of this Note to exercise,
and no delay in exercising, any right or remedy hereunder or under any other
document, instrument or agreement shall constitute a waiver thereof on the part
of any holder; nor shall any single or partial exercise of any right or remedy
hereunder or under any other document, instrument or agreement preclude any
other or further exercise thereof or the exercise of any other right or remedy.

      It is the intention of the parties hereto to conform strictly to
applicable usury laws regarding the use, forbearance or detention of the
indebtedness evidenced by this Note whether such laws are now or hereafter in
effect, including the laws of the United States of America or any other
jurisdiction whose laws are applicable, and including any subsequent revisions
to or judicial interpretations of those laws, in each case to the extent they
are applicable to this Note (the "Applicable Usury Laws"). Accordingly, if any
acceleration of the maturity of this Note or any payment by Maker or any other
person or entity produces a rate in excess of the Highest Lawful Rate or results
in Maker or such other person or entity being deemed to have paid any interest
in excess of the Maximum Amount, as hereinafter defined, or if any holder hereof
shall for any reason receive any unearned interest in violation of any
Applicable Usury Laws, or if any transaction contemplated hereby would otherwise
be usurious under any Applicable Usury Laws, then, in that event,
notwithstanding any other provision of this Note or any other agreement or
instrument, it is agreed as follows: (i) the provisions of this paragraph shall
govern and control; (ii) the aggregate of all interest under Applicable Usury
Laws that is contracted for, taken, charged, collected, reserved or received
under this Note or otherwise shall under no circumstances exceed the Maximum
Amount, and any amount that would be excessive interest 

                                 Page 2 of 3
<PAGE>
shall be applied to the reduction of the principal amount owing in respect of
this Note and not to the payment of interest, or if such excessive amount
exceeds the principal amount owing in respect of this Note, any excess shall be
promptly refunded to the Maker or such other person or entity by the holder
hereof; (iii) neither Maker nor any other person or entity shall be obligated to
pay the amount of such interest to the extent that it is in excess of the
Maximum Amount; and (iv) the effective rate of interest on this Note shall be
ipso facto reduced to the Highest Lawful Rate, as hereinafter defined, and the
provisions of this Note immediately shall be deemed reformed, without the
necessity of the execution of any new document or instrument, so as to comply
with all Applicable Usury Laws. All sums paid, or agreed to be paid, to any
holder hereof for the use, forbearance or detention of the indebtedness of the
Maker evidenced by this Note shall, to the fullest extent permitted by the
Applicable Usury Laws, be amortized, pro rated, allocated and spread throughout
the full term of the indebtedness evidenced by this Note. As used herein, the
term "Maximum Amount" means the maximum nonusurious amount of interest which may
be lawfully contracted for, taken, charged, collected, reserved or received by
the holder hereof in connection with the indebtedness evidenced by this Note
under all Applicable Usury Laws, and the term "Highest Lawful Rate" means the
maximum rate of interest, if any, that may be contracted for, taken, charged,
collected, reserved or received under all Applicable Usury Laws on the principal
balance of this Note from time to time outstanding.

      THIS NOTE SHALL BE SUBJECT TO AND  CONSTRUED  AND ENFORCED IN ACCORDANCE
WITH THE  INTERNAL  LAWS OF THE STATE OF TEXAS  WITHOUT  GIVING  EFFECT TO ANY
CONFLICTS OF LAWS PRINCIPLES.

      IN WITNESS  WHEREOF,  each of the undersigned has caused this Note to be
executed in Austin, Texas on the date first above appearing.

 


                                          JOHN G. VONDRAK
                                          John G. Vondrak, Individually

                                  Page 3 of 3