Joel and Octavio Angulo, Fidel Vargas and Jaime Limdn were convicted following a second jury trial1 for conspiracy to distribute cocaine and/or heroin and for possession with intent to distribute cocaine and/or heroin in violation of 18 U.S.C. §§ 841(a)(1) and 846. We now remand to the district court for an evidentiary hearing relating to the defendants’ motion for mistrial based upon claims of jury tampering. We find no merit to the other claims raised on appeal.

Limón asserts several evidentiary errors. These arguments are without merit.2 We also find no merit in Limon’s argument that the district court abused its discretion in denying his motions to sever and for a new trial and erred in denying his Rule 29 motion for acquittal based on insufficiency of the evidence.3 We reject as well the argument by defendants Joel and Octavio Angulo that the district court erred in denying them downward adjustments based upon acceptance of responsibility.4

*846We turn now to the question of jury tampering.. During the course of the trial, a juror, Ms. Zodolske, received a telephone call at her home in which the caller said, “I know where you live.” The next day, Ms. Zodolske asked the other jurors if they had received a strange phone call. She then explained the circumstances of the call to them, and several of them encouraged her to inform the judge. After bringing the issue to the judge’s attention, Ms. Zodolske was called into the judge’s chambers with the court reporter present' but not the attorneys. The judge asked her about the call. She informed the judge that she had told the other jurors about the call. She admitted that she was scared and that she probably would not want twelve jurors like herself to sit on a case if she were the defendant. The judge thereafter excused Ms. Zodolske from the jury.

Later, the following dialogue with court and counsel took place:

THE COURT: She merely asked the other jurors if they had received such a call. They said no. That juror, if you will recall, also is the one who has the wedding in Davis on the 13th. She was not completely sure she could be fair. I concluded on the basis of my conversation that the other jurors were not affected, ... and I dismissed her.

MR. CHAVEZ: Your Honor, then she did convey that to the other jurors.

THE COURT: Mr. Chavez.

MR. CHAVEZ: Yes, sir.

THE COURT: She said she asked the other jurors if they received any strange calls last night. It’s all on the reporter’s transcript. If you want to buy a transcribed version, you may do so.

The judge then abruptly denied defense counsel’s motion for a mistrial, stating, “All motions made or anticipated are denied.”5 He did not explain.Ms. Zodolske’s sudden absence to the other jurors, did not question the remaining jurors about possible bias, and did not give a curative instruction.

In Remmer v. United States, 347 U.S. 227, 74 S.Ct. 450, 98 L.Ed. 654 (1954), the Supreme Court remanded to the district court, some two years after the trial, to hold an evidentiary hearing based upon a defendant’s motion for a new trial to determine whether improper contact with a juror was harmless to the defendant. Id. at 229, 74 S.Ct. at 451. The Court observed that there exists a presumption of prejudice which the government must rebut in cases involving jury tampering:

In a criminal case, any private communication, contact, or tampering, directly or indirectly, with a juror during a trial about the matter pending before the jury is, for obvious reasons, deemed presumptively prejudicial, if not made in pursuance of known rules of the court and the instructions and directions of the court made during the trial, with full knowledge of the parties. The presumption is not conclusive, but the burden rests heavily upon the Government to establish, after notice to and hearing of the defendant, that such contact with the juror was harmless to the defendant.

Id. (citations omitted).

The Remmer Court found that the trial judge’s failure to conduct an evidentiary hearing at the time was error. Id. at 229-30, 74 S.Ct. at 451. It therefore reversed the court of appeal’s holding that the district court did not abuse its discretion in failing to hold a hearing, observing as follows:

The integrity of jury proceedings must not be jeopardized by unauthorized invasions. The trial court should not decide and take final action ex parte on information such as was received in this case, but should determine the circumstances, the impact thereof upon the juror, and whether or not it was prejudicial, in a hearing with all interested parties permitted to participate.

Id.

In the Remmer case, only one juror was involved, but that juror was allowed to remain on the venire. In the present case, Ms. Zodolske told other jury members about the threatening telephone call. The judge then removed Ms. Zodolske from the jury, an event obviously noticed by the remaining ju*847rors. Under these facts, the government has a difficult burden to overcome the obvious possible prejudice. Yet here, as in Remmer, the trial judge eonclusorily found no prejudice without holding any hearing whatsoever.

It is true, of course, that not every improper ex parte contact with a juror requires a mistrial. As the Seventh Circuit has noted, “Our system of justice has not delegated to every reprobate the power to effect a mistrial.” United States v. Williams, 737 F.2d 594, 612 (7th Cir. 1984), cert. denied, 470 U.S. 1003, 105 S.Ct. 1354, 84 L.Ed.2d 377 (1985). However, the Supreme Court has stressed that the remedy for allegations of jury bias is a hearing, in which the trial court determines the circumstances of what transpired, the impact on the jurors, and whether or not it was prejudicial. See Remmer v. United States, 347 U.S. 227, 229-30, 74 S.Ct. 450, 98 L.Ed. 654 (1954); Smith v. Phillips, 455 U.S. 209, 216, 102 S.Ct. 940, 945, 71 L.Ed.2d 78 (1982).

An evidentiary hearing is not mandated every time there is an allegation of jury misconduct or bias. See, e.g., United States v. Langford, 802 F.2d 1176, 1180 (9th Cir. 1986), cert. denied, 483 U.S. 1008, 107 S.Ct. 3235, 97 L.Ed.2d 740 (1987). Bather, in determining whether a hearing must be held, the court must consider the content of the allegations, the seriousness of the alleged misconduct or bias, and the credibility of the source. See Hard v. Burlington N.R.R., 812 F.2d 482, 485 (9th Cir. 1987).

Considering these factors, we deem it clear that the district court abused its diseretion in failing to hold a hearing under the facts presented in this case. Ms. Zodolske informed the judge that she had received a threatening phone call and that she had told the other jurors about it. The judge excused Ms. Zodolske from the jury, stating that “[p’Jeople don’t go through life receiving calls like this ... and the most logical activity for you to tie it up with is jury duty.”6 However, the trial judge never explained Ms. Zo-dolske’s absence to the other jurors, who knew she had received the threatening telephone call, nor did he question the other jurors about what effect the threat to Ms. Zodolske and her subsequent dismissal from the venire had on them. Under these facts, the remaining jury members may well have believed that defendants were responsible for the threat and, based on that assumption, may have decided the merits of the case on that basis. See United States v. Shapiro, 669 F.2d 593, 601 (9th Cir. 1982) (court must guard against jury assuming one of the parties was responsible for an attempted jury tampering); United States v. Williams, 737 F.2d 594, 613 (7th Cir. 1984) (by talking with jurors, judge acted to avoid “alarming possibility” that jurors would mistakenly believe threatening call was from either party).

Here, the potential for bias is so strong that the judge was obliged at a minimum to hold a hearing.7 In cases where a bribe or a threat to a juror was communicated to the other jurors, the trial judge must fully examine the effect of the threat on the remaining jurors. See, e.g., United States v. Sublet, 644 F.2d 737, 740-41 (8th Cir. 1981) *848(following comment by person in courtroom to one juror, who then relayed it to four other jurors, district court held hearing with all affected jurors); United States v. Norton, 700 F.2d 1072, 1076 (6th Cir.) (following threatening telephone conversation to juror which was communicated to other jurors, judge held hearing with all affected jurors), cert. denied, 461 U.S. 910, 103 S.Ct. 1885, 76 L.Ed.2d 814 (1983). This is not to say that ordering a mistrial must necessarily follow. See Sublet, 644 F.2d at 741 (comment to juror was not viewed by any jurors as frightening; curative instruction was properly done); Norton, 700 F.2d at 1076 (jurors viewed call as “harmless prank” and were not swayed by the contact). However, without a hearing, a district court does not have the essential facts to properly evaluate a defendant’s motion for mistrial.8

The government suggests that the need for a hearing was somehow negated by defense counsel’s failure to specifically request one. We disagree. First, it is clear from the case law that the only motion defendant need make to trigger the need for a hearing is a motion for a new trial or mistrial, which defendants made in this case. Upon that motion, the district court was obliged to inquire as to the circumstances, determine whether the affected jurors remained impartial, and then decide whether the mistrial should be granted. See Smith v. Phillips, 455 U.S. 209, 212-13, 102 S.Ct. 940, 943-44, 71 L.Ed.2d 78 (1982) (evidentiary hearing properly held on question of jury bias upon defendant’s motion to set aside the verdict). Indeed, in Smith v. Phillips, the Court stressed that “a trial judge [must be] ever watchful to prevent prejudicial occurrences and to determine the effect of such occurrences when they happen.” Id. at 217, 102 S.Ct. at 946. Because even a single partial juror violates a defendant’s constitutional right to a fair trial, a judge must always act to diligently preserve that right.9

Accordingly, we remand to the district court to hold an evidentiary hearing to determine whether the jurors who knew of the threat were able to act impartially and without bias. The government will be required to show that the threatening telephone call was harmless beyond a reasonable doubt to defendants. United States v. Littlefield, 752 F.2d 1429, 1431-32 (9th Cir. 1985). If the government cannot make that showing, a new trial should be ordered.

IT IS SO ORDERED.