Anthony Casella appeals from a judgment convicting him of four counts of theft by unauthorized taking, 17-A M.R.S.A. § 353(1) (1983), and one count of witness tampering, 17-A M.R.S.A. § 454(1)(A)(2) (Supp.1992), entered after a jury trial in the Superior Court (Cumberland County, Perkins, J.). Casella contends that he was deprived of a fair trial because the State repeatedly referred to him as a “liar” during its closing arguments at the trial. We agree and vacate the judgments.
Through his business, Case Equipment, Inc., Casella sold the “Sublicolor System,” a machine that transposed images onto T-shirts, baseball caps, and coffee mugs. The charges in question are based on four separate transactions in which Casella, inter alia, refused to return a down payment as promised; promised to sell a machine that Case Equipment neither manufactured nor sold; and promised to hold a customer’s check while the customer contemplated purchasing a machine but promptly negotiated the check despite his promise.
During his closing argument, the prosecutor, not less than forty-one times, asserted his opinion that Casella had lied. On at least seven of those occasions, the prosecutor’s view related to Casella’s in-court testimony. For example, the prosecutor stated:
You were never supposed to hear him explain his lies. He didn’t count on sitting in that [witness] chair.
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... [T]hat’s how you tell that Anthony Casella meant to steal because remember what I said he’ll say anything_ He’ll tell any lie that has to be told to get and keep the money. And that includes lying *122to his attorneys and lying to you on that witness stand.
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Defendant testified, ladies and gentlemen, about a computer portrait system that he said he ordered for Helen Piante-dosi in 1988 from Kokomo, Indiana.... [T]hat story was hogwash. Anthony Ca-sella made it up....
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Anthony Casella and A1 Daniel couldn’t keep their lies straight....
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Remember what I told you. He will tell any lie, any lie to get what he wants and that includes lying to you....
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He is truly an accomplished liar, ladies and gentlemen. He is [a] master salesman. And he is giving you the sales pitch of his life. And you are the consumers. He thinks that he can sell you his pack of lies. He thinks he can talk his way out of this. He thinks he can sell you a bill of goods. Don’t buy what Anthony Casella is trying to sell you. Don’t be his next victim. Don’t let him lie to you and get away with it.
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Don’t let him walk out of this courtroom thinking he could con you. You have watched him and you have listened to him and you have taken the measure of Anthony Casella. Now, it’s time for you to do your duty and convict him of the crimes he committed. Thank you.
Immediately following the prosecutor’s closing argument to the jury, defense counsel moved for a mistrial. The trial court denied the motion. We review the court’s decision to deny the motion for mistrial only for an abuse of discretion. State v. Harnish, 560 A.2d 5, 9 (Me.1989).
On appeal, the State acknowledges that “the prosecutor attacked Casella’s credibility by suggesting that Casella lied to the jury while on the witness stand” but argues that his “characterization of the defendant’s testimony as ‘lies’ ... was not an expression of personal opinion.” We disagree. The prosecutor may well believe in the correctness of his opinion, and his belief may even be well founded but it is an opinion nonetheless.
We have repeatedly held that it is improper for a prosecutor to express an opinion on the credibility of a defendant. See, e.g., State v. Pendexter, 495 A.2d 1241, 1241 (Me.1985) (“Our rules and opinions make clear that it is improper for a prosecutor to express his personal opinion on the credibility of a defendant or of a witness.”); State v. Smith, 456 A.2d 16, 17 (Me.1983) (“Although the prosecutor may properly attack defendant’s credibility by analyzing the evidence and highlighting absurdities or discrepancies in defendant’s testimony, and may present his analysis in summation with vigor and zeal, he may not properly convey to the jury his personal opinion that a defendant is lying.”); and State v. Reilly, 446 A.2d 1125, 1129 (Me.1982) (finding the prosecutor’s comment that defense counsel “knew [the defendant] was a liar ... particularly damaging”); see also M.Bar R. 3.7(e)(2)(v) (“In appearing in a professional capacity before a tribunal, a lawyer shall not ... [a]ssert a personal opinion as to the ... credibility of a witness ”). We have stated the reason for this rule as follows:
The policies underlying this proscription go to the heart of a fair trial. The prosecutor is cloaked with the authority of the [State]; he stands before the jury as the community’s representative. His remarks are those, not simply of an advocate, but rather of a [State] official duty-bound to see that justice is done.
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... [T]he representative of the government approaches the jury with the inevitable asset of tremendous credibility — but that personal credibility is one weapon he must not use.
State v. Smith, 456 A.2d at 18 (citations omitted).
We have recently reiterated this rule. See State v. Steen, 623 A.2d 146, 149 (Me.1993). In Steen, we vacated a gross sexual assault conviction on the ground that the prosecutor impermissibly asserted to the jury her per*123sonal belief that the defendant and other witnesses were lying,1 stating that:
Although it is proper for the State to point out inconsistencies in a defendant’s statement, it is impermissible for a prosecutor to assert that the defendant lied on the stand. The prosecutor through her cross-examination of Steen and her statements in closing remarks clearly suggested to the jury that she thought these witnesses were lying.
Id. (citations omitted).
The State cites State v. Pendexter, 495 A.2d at 1241, in which we held that “not all references to the credibility of the defendant ... are improper” and that “[t]he central question is whether the comment is fairly based on the facts in evidence,” and argues that in this case its closing argument was based on the facts in evidence and thus was not improper. We disagree. In Pendexter, a witness freely admitted that he had stolen the vehicle that was the subject of the case and acknowledged that he had subsequently lied to the owner of the vehicle when confronted with the accusation. Id. We concluded that, under those circumstances, the prosecutor’s comments to the jury, in which he referred to that witness as an admitted thief and an admitted liar, were adequately based on the facts in evidence. Id. Here, Casella made no such admission. It was for the jury, not the prosecutor, to determine which witnesses were telling the truth. See State v. Crocker, 435 A.2d 58, 77 (Me.1981) (“Determining what credence to give to the various witnesses and their testimony is a matter within the exclusive province of the jury.”).
The State acknowledges that in State v. Smith, 456 A.2d at 16, we vacated an assault conviction where the prosecutor similarly conveyed a personal belief that the defendant had lied on the stand.2 The State nevertheless attempts to distinguish Smith on the basis that Smith involved assault charges, whereas “Casella’s pattern of deception with respect to his victims was central to the State’s case.” The fact that the charges are based on deception does not mean that the prosecutor is therefore free to express his personal opinion that the defendant was lying in court. A rule allowing the prosecutor to express his opinion on the credibility of the defendant’s testimony in cases where the defendant’s veracity is central to the prosecution has no precedent in our jurisprudence. In fact, when credibility is a crucial issue, prosecutorial comments attacking a defendant’s credibility can be especially damaging and deserving of sharp scrutiny. See, e.g., State v. Smith, 456 A.2d at 19 (“The credibility of the participants in the fight was crucial, and the prosecutor’s improper remarks went directly to that issue.”); State v. Reilly, 446 A.2d at 1130 (“The prosecutor’s improper comments went directly to credibility—the principal underlying issue at trial.”).
The only question remaining in this case is whether the prosecutor’s improper comments to the jury resulted in harmless error. See M.R.Crim.P. 52. We should vacate the conviction unless we believe “it [is] highly probable that the error did not affect the judgment.” State v. True, 438 A.2d 460, 467 (Me.1981). In the case now before us, the prosecutor, throughout his closing argument, repeatedly told the jury that Casella had lied to them. The prosecutor urged the jury not to be taken in by Casella’s testimony which he characterized as a “pack of lies” and a “bill of goods.” He concluded his *124argument to the jury by telling them that they should not “let Casella walk out of this courtroom thinking he could con you,” that they should “do [their] duty and convict him,” and avoid becoming “his next victim[s].” Because credibility in large measure determined the ultimate issue of guilt in this case, and because the prosecutor’s improper comments went directly to credibility, see State v. Reilly, 446 A.2d at 1130, we cannot say that it is “highly probable that the jury’s determination ... was unaffected by the prosecutor’s comments.” State v. Steen, 623 A.2d at 149.
The entry is:
Judgment vacated.
ROBERTS, GLASSMAN and COLLINS, JJ., concurring.
. In Steen, the prosecutor’s offensive comments included the following questions to the defendant: "So you made this one up on your own?” and "So now we have three versions ... It is hard for you to keep this all straight, isn't it?” State v. Steen, 623 A.2d at 149. We also found that the prosecutor’s comments on the testimony of the defendant’s expert witness made during her closing argument were improper. The prosecutor stated: “I suggest to you, ladies and gentlemen, that [the defendant’s expert's] opinion is based on $2,500, the money the defendant paid him for his testimony.” Id. at 149.
. In Smith, the prosecutor commented to the jury that "[the defendant] gets caught lying and he can’t even admit it,” "You people are smarter than that, to let him come in here and tell you he was telling the truth because he wasn’t,” and “You tell Philip Smith if he should be accountable for what he did and held responsible for what he did or you tell him it’s okay to lie.” State v. Smith, 456 A.2d at 17.