This appeal is concerned with a contractual dispute arising from an agreement for painting of the Kyle office building in Oak Park, Michigan. The defendant, Alfred A. Smith Company, was engaged as general contractor in the construction of the Kyle building. The plaintiff, Ensign Painting Company, was a subcontractor employed by defendant to paint the building in question.
Testimony taken during the trial, over the objections of defendant, indicated that during the spring of 1965, a meeting was held between representatives of the general contractor and the plaintiff regarding prices for the painting of the Kyle building. During this discussion, the job plans were examined to determine the scope of. plaintiff’s work. The plans indicated that prices submitted for the painting trade should include an amount for painting approximately 2,000 lineal feet of dry wall stud partitions to be erected in the proposed tenant areas of the building. The plaintiff submitted a bid of $5,225, and the parties orally agreed that this price included only the painting of exterior walls and stairwells and specifically excluded any figure for the partition work in the tenant areas.
Approximately two weeks later, a purchase order dated April 29, 1965, was sent by defendant to the plaintiff which stated:
“Furnish necessary labor, material, equipment, and services and perform all work required in the general conditions to furnish, install and complete *499the painting and decorating work — pages 1501-1505 —all in accordance with plans and specifications and addendums 1, 2 and 3 prepared by Seymour J. Levine, A. I. A. Architects — Job No. 6137 for the sum of Five Thousand Two Hundred Twenty-Five Dollars. Sales Tax included. Payments to be made in accordance with article 37 A. I. A. Document A-201. Entire project to be completed October 1, 1965.”
The plans and specifications mentioned in the purchase orders were mailed to the plaintiff on May 12, 1965. These specifications as well as the plans indicated that the contract was to include the painting of the exterior walls, stairwells and 1,820 lineal feet of dry wall stud partitions.
The plaintiff commenced work on the Kyle job during the spring of 1965, and in February, 1966 had completed the paint work, with the exception of the interior partitions. During the same month, a written billing invoice was sent to defendant stating that its obligation had been performed in accordance with the purchase order of April 29, 1965, and the plans and specifications referred to therein. Plaintiff encountered considerable difficulty in securing payment and the matter was finally turned over to an attorney who collected $5,225.
Later, it became apparent that additional paint work would be required to complete the project. In June of 1966, plaintiff submitted a bid for the additional work and defendant issued a second purchase order to allow completion of the Kyle job. The plaintiff completed most of the work and billed defendant for the amount. Defendant refused to pay several of the invoices, alleging that they were being charged for work plaintiff was already obligated to complete in accordance with the original purchase order of April 29, 1965. Plaintiff then brought suit *500in the Oakland county circuit court to collect the amount due on the Kyle building as well as unpaid invoices resulting from other projects.
The court held that the purchase order of April 29, 1965, and the plans and specifications referred to therein superseded all oral understandings between the parties. The court refused to consider oral testimony which would vary the written terms of the purchase order, which was held to be a valid unilateral contract, binding upon both parties when substantial performance had been accomplished.
Plaintiff was given judgment of $1,202.14, the amount of unpaid invoices independent of the Kyle project. Defendant was granted a set-off of $654.94 for money paid to Villaire Painting Services for work the plaintiff should have done on the project, leaving a net judgment for plaintiff of $547.20.
' It is the contention of plaintiff that there was a valid oral contract between the parties for the painting, specifically excluding the disputed partition work, and the purchase order of April 29, 1965, was merely an unaccepted counter-offer submitted by defendant. The defendant maintains that there existed a valid written contract between the parties as evidenced by the purchase order and therefore- oral testimony is inadmissible to vary or contradict the terms of a written contract which has been accepted and performed by both parties.
The main issue with which we are faced on this appeal is whether the purchase order of April 29,1965, constituted a valid contract which superseded all prior oral contracts. We are also asked to consider whether the subsequent purchase order of defendant, dated August 2, 1966, alters or amends the terms of the first order, thus creating a new and independent contract governing the painting of interior partition work.
*501It is the holding of this Court that the purchase order dated April 29,1965, constituted a valid contract superseding all prior oral agreements between the parties. This position is explained and upheld by Shannon v. Polish Falcons of America, Nest No. 86 (1957), 348 Mich 92, 105, quoting 12 Am Jur, Contracts, § 233, p 757:
“No parol agreement prior to, or contemporaneous with, a written contract, which tends to vary or contradict either its express provisions or its legal import thereto, can be considered in interpreting it. A parol agreement inconsistent with a written agreement made contemporary therewith is void. Parol understandings, although they induce the making of a written contract, are merged in the writing so that they cannot be used to change the contract or show any intent different from that expressed in the instrument.”
No ambiguity exists in the questioned purchase order and we find no indication of fraud, duress or mistake. The order specifically provides for plaintiff’s completion of the paint work on the Kyle office building in accordance with pages 1501-1505 of the job specifications and as noted in the job plans. The order contains all of the necessary elements of a contract except that it was not executed by both of the parties. However, in the spring of 1965, plaintiff began performance of the order and substantially completed the required work on February 28, 1966.
The case of Cohen v. Jackoboice (1894), 101 Mich 409, is in point, for it deals with enforcement of a unilateral contract in which one of the parties had undertaken performance of the obligation under a written agreement while the second party was attempting by way of parol evidence to show that, under a prior oral agreement, the contract could be terminated at his will. The court stated;
*502“There is no ambiguity in this instrument. It is an order to insert an advertisement, to occupy a given space, for a given period, at a given price, and specifies when the price agreed upon shall be paid. It contains all the indicia of a contract, except that it wasn’t executed by both parties. Defendant does not attempt to supply an omission in the instrument. Tie does not undertake to show any fraud or misrepresentation in matters of inducement.”
The court ultimately held that “the instrument * * * is more than a mere memorandum. It has all the elements of an agreement, and the admission of parol testimony tending to contradict terms which are plain and explicit was erroneous.” In an analysis of the case cited, supra, and 17 CJS, Contracts, § 59, p 729 it is apparent that the rule outlined in the Polish Falcon case, supra, is not limited to formally written bilateral agreements, but also has been extended to a unilateral written offer by one party where the other party undertakes performance pursuant to that particular offer. In the case at bar, the plaintiff, having undertaken substantial performance under the unilateral written order, accepted the entire order as a contract.
The existence of a written contract embodied in the terms of the April 29, 1965, purchase order is further exemplified and conceded by a billing invoice, dated February 28,1966, sent by plaintiff to defendant. The bill makes specific reference to the aforementioned purchase order and states that it is for:
“Labor, materials and equipment to paint and decorate at the Kyle office building in accordance to [sic] plans and specifications.” (Emphasis supplied.)
As mentioned, supra, both the plans and specifications specifically provide that the work which is the subject of this dispute is to be painted and included *503as part of the based bid. The order was therefore a unilateral contract and acceptance occurred when plaintiff had substantially completed performance. Notice of this acceptance was unnecessary since the offer in question called for performance, and notice of acceptance was not a condition dictated in the purchase order.
Since we have held that the original purchase order was a binding written contract, it cannot be varied or contradicted through introduction of parol evidence, unless there has been a showing of fraud, duress or mistake, and no such allegation was either made or proved in the case at bar. See Hawheye Casualty Co. v. Frisbee (1947), 316 Mich 540, Getter v. Levine (1946), 315 Mich 353, and Danto v. Charles C. Robbins, Inc., (1930), 250 Mich 419. The words used by the trial court summarize the basis of our opinion and serve to explain our reasoning when it stated:
“I think obviously this is a written instrument which on its face is not ambiguous, nor is there any fraud alleged, and consequently the written agreement was to prevail. Payment has been made on the written agreement.”
It is further argued by the plaintiff that even if the written purchase order dated April 29,1965, was a binding contract, the parties could and actually did change it. Such cases as Bishop Electric, Inc., v. Simpson (1967), 7 Mich App 391, and Green v. Millman Brothers, Inc. (1967), 7 Mich App 450, are cited in support of this position. While we do not question the principle set forth, these cases are not applicable to the facts in the case at bar. The plaintiff argues that defendant’s second purchase order dated August 2, 1966, called for the painting of dry wall partitions and was, in effect, an entirely new *504agreement superseding the original contract. We reject this line of reasoning, as it is entirely without a logical or factual basis.
The subsequent purchase order of August 2, 1966, specifically stated:
“Unit prices for additional worh at the Kyle office building.”
The order does not contain language which either alters or amends the original order, nor is reference even made to its predecessor.
Judgment of the lower court affirmed. Costs to appellees.
T. M. Burns, J., concurred.