Opinion by Judge PREGERSON; Dissent by Judge WEXLER.

Rogelio Banuelos-Rodriguez pled guilty to illegally reentering the United States after being deported, in violation of 8 U.S.C. § 1326 . He was sentenced to a prison term of seventy months. At sentencing, he argued for a downward departure from the applicable range under the Sentencing Guidelines. He contended that he was entitled to downward departure because of a sentencing disparity that existed between the Central District of California and the Southern District of California for a violation of § 1326. He asserted that if he had been prosecuted in the Southern District of California, the Government would have entered into a plea agreement with him that would have resulted in a sentence of only twenty-four months.

The district court held that sentencing disparity between federal districts for a violation of § 1326 is not a proper basis for downward departure. We disagree.

We hold that sentencing disparity for a § 1326 violation that arises from different plea-bargaining policies of United States Attorneys in California’s Central and Southern Districts can be a valid basis for downward departure. Thus, we reverse and remand.

FACTS AND PRIOR PROCEEDINGS

In August 1995, a federal grand jury returned a one-count indictment charging Banuelos-Rodriguez with being a deported alien found in the Central District of California in violation of 8 U.S.C. §§ 1326 (a), (b)(1), and (b)(2). 1

Banuelos-Rodriguez pled guilty tó the indictment. In accordance with his plea agreement, Banuelos-Rodriguez admitted he was guilty of violating § 1326(a) and further admitted facts that made him subject to the sentencing enhancement provisions of § 1326(b)(2). At sentencing, he argued for a downward departure from the sentencing guideline range based on an alleged disparity in the severity of sentences given to aliens convicted of § 1326 violations in different federal districts of California. In support of his motion for downward departure, Banuelos-Rodriguez submitted an article from the Los Angeles Daily Journal, a legal newspaper. The article stated that a previously deported alien indicted in the Southern District of California would be eligible for that District’s “fast track” plea-bargaining program. See Thom Mrozek, Prosecutions on the Rise: U.S. Attorneys Take Varying Approaches to Illegal Re-Entry, L.A. Daily J., Sept. 21, 1995, at 1. According to the article, a defendant’s participation in the “fast track” program significantly reduces his or her sentencing range in exchange for a timely guilty plea. Id. at 9. The article states that “the vast majority of defendants-except those convicted of the most violent and depraved aets-are offered a deal under Section 1326(a), which carries a statutory maximum sentence of two years in prison.” Id. The article also states that “[tjhose few defendants who face longer prison terms under [§ ] 1326(b) are offered plea bargains that see their sentences top out at five years.” Id. at 9. According to the article, California’s Eastern and Northern Federal Districts *743 have adopted policies similar to the Southern District’s “fast track” program. Id.

The district court denied Banuelos-Rod-riguez’s motion for a downward departure and stated: “[I]f the court accepts whatever has been bargained and then sentences pursuant to the guidelines, then I don’t see how disparity in the plea bargain charging [among] the various districts adds up to a downward departure factor.” The court concluded “it’s not a proper factor for departure.”

The district court sentenced Banuelos-Rodriguez to seventy months of imprisonment and three years of supervised release. Banuelos-Rodriguez timely appeals this sentence. We have jurisdiction under 18 U.S.C. § 3742 (a) and 28 U.S.C. § 1291 .

ANALYSIS

I. Standard of Review

“[A] unitary abuse of discretion standard governs our review of departure decisions.... ” United States v. Sablan, 114 F.3d 913, 916 (9th Cir.1997) (en banc). While we defer to a district court’s discretion in departure decisions, a “ ‘district court by definition abuses its discretion when it makes an error of law.’ ” Id. (quoting Koon v. United States, 518 U.S. 81, 99 , 116 S.Ct. 2035, 2047 , 135 L.Ed.2d 392 (1996)).

II. Factors Considered in Departure Decisions

In Koon , the Supreme Court described the factors that a district court should consider when confronted with a motion to depart downward from the applicable Sentencing Guideline range:

518 U.S. at 95 , 116 S.Ct. at 2045 (quoting United States v. Rivera, 994 F.2d 942, 949 ([1st Cir.] 1993)). The district court in the present case did not consider these factors when confronted with Banuelos-Rodri-guez’s motion for downward departure. Our review of these factors convinces us that sentencing disparities among federal districts based on U.S. Attorneys’ plea-bargaining practices can be a ground for departure in the appropriate case.

We first note that a disparity in sentencing among federal districts arising from plea-bargaining policies of U.S. Attorneys is not one of the factors forbidden by the Commission. See United States Sentencing Commission, Guidelines Manual, § 5H1.10, p.s. (Nov.1995) (forbidding “Race, Sex, National Origin, Creed, Religion, and Socio-Economic Status” as grounds for sentencing departure). Nor is such a disparity specifically “discouraged” by the Sentencing Guidelines. See Koon, 518 U.S. at 95 , 116 S.Ct. at 2045 (discussing specific offender characteristics that are discouraged as bases for departure under the Sentencing Guidelines). On the other hand, the sentencing disparity among districts due to plea-bargaining policies cannot be described as an “encouraged” basis for departure in the Sentencing Guidelines. See, e.g., U.S.S.G. § 5K2.13, p.s. (encouraging departures based on a defendant’s “significantly reduced mental capacity”).

Disparity in sentencing among federal districts arising from the plea-bargaining policies of U.S. Attorneys is simply not a factor mentioned in the Sentencing Guidelines. Thus, “[we] must, after considering the structure and theory of both relevant individual guidelines and the Guidelines taken as a whole, decide whether it is sufficient to take the case out of the Guide *744 line’s heartland.” Koon, 518 U.S. at 96 , 116 S.Ct. at 2045 (internal quotations and citation omitted).

A. Sentencing Guideline § 2L1.2

The structure and theory of U.S.S.G. § 2L1.2 (the relevant individual guideline) suggest that a downward departure may be appropriate in this case. To understand § 2L1.2 and the Southern District’s “fast track” program, it is helpful to first examine the underlying criminal statute. Banuelos-Rodriguez was charged with a violation of 8 U.S.C. § 1326 , which provides in part:

8 U.S.C. § 1326 .

The sentencing exposure for an alien varies dramatically depending on whether subsection (b) of § 1326 is applied. In cases such as this one, that variance depends on whether the defendant is required, as part of his plea to § 1326(a), to admit to facts that subject him to the sentencing enhancement provisions under § 1326(b). •

U.S.S.G. § 2L1.2.

Thus, as was the case in Koon , “[t]he heartland of [the guideline] will vary depending on the defendant’s conduct.” Koon, 518 U.S. at 101 , 116 S.Ct. at 2048 . In general, prior felony convictions result in an increase of four levels. U.S.S.G. § 2L1.2(b)(l). Convictions for “aggravated” felonies result in an increase of sixteen levels. Id. at § 2L1.2(b)(2) and comment. (n.7) (defining “aggravated felony”).

The theory behind this guideline is manifest in its structure: the more extensive a reentering alien’s criminal history, the more severe the sentence. The structure and theory of § 2L1.2 do not take into account the fact that sentencing disparity for § 1326 violations might occur among federal districts because of different plea-bargaining practices of U.S. Attorneys. While the Guideline calls for longer sentences for aliens with more severe criminal histories, in practice, given the alleged sentencing disparity, the single most influential factor in an alien’s sentencing for a violation of § 1326 is the location of his arrest. In short, an alien who crosses the *745 invisible county line separating the Southern and Central Districts (the districts comprise a contiguous, heavily populated metropolitan area) may more than triple his or her federal sentence for violating § 1326. The “heartland” of Guideline § 2L1.2 certainly does not take account of that variable.

A district court’s consideration of such disparity as a basis for downward departure is consistent with the Guideline’s central structural premise that an extensive criminal history-not the location of arrest-will result in a more severe sentence. The structure and theory of § 2L1.2 could support a finding that Banuelos-Rodriguez’s case is not within the heartland of this guideline, thus warranting a downward departure.

B. Sentencing Guidelines Taken as a Whole

Our examination of the structure and theory of the Sentencing Guidelines taken as a whole also leads us to conclude that this case may be outside of the Guidelines’ “heartland,” again warranting downward departure.

A central goal of the Sentencing Guidelines is to eliminate sentencing disparity. The purpose of the Sentencing Commission was to establish guidelines that “avoid[ ] unwarranted sentencing disparities among defendants with similar records who have been found guilty of similar criminal conduct.” 28 U.S.C. § 991 (b)(1)(B). As the Commission explained, “Congress sought reasonable uniformity in sentencing by narrowing the wide disparity in sentences imposed for similar criminal offenses committed by similar offenders.” U.S.S.G. Ch. 1, Pt. A, p.s. 3. “The goal of the Sentencing Guidelines is, of course, to reduce unjustified disparities and so reach towards the even-handedness and neutrality that are the distinguishing marks of any principled system of justice.” Koon, 518 U.S. at 113 , 116 S.Ct. at 2053 .

Federal sentences that triple in severity solely because of the federal district in which the alien is convicted frustrate congressional intent to secure reasonable uniformity in sentencing. In this case, similar, contiguous federal districts employ radically different plea bargaining policies in otherwise-similar cases. The disparity in sentences, the similarities of the districts involved, and the close proximity of those districts are factors that persuade us that a downward departure may be warranted in this case.

The Government argues that because Banuelos-Rodriguez has made no showing of impermissible discrimination, the charging policy of the U.S. Attorney for the Central District is a valid exercise of pros-ecutorial discretion. See United States v. Bauer, 84 F.3d 1549, 1560 (9th Cir.1996) (“A prosecutor’s decision to bring charges is beyond judicial review unless the defendants can make a prima facie showing that the decision rested on an impermissible basis.”).

In light of the Supreme Court’s decision in Almendarez-Torres v. United States, 118 S.Ct. at 1226, that § 1326(b)(2) does not describe a separate criminal offense from § 1326(a), but rather provides a sentencing enhancement, the differential treatment of reentering aliens is a result of plea-bargaining, not charging, policies.

Moreover, while we acknowledge the prosecutor’s discretion in adopting plea-bargaining policies, we nonetheless are mindful that the Sentencing Guidelines “are intended to ensure that plea negotiation practices ... do not perpetuate unwarranted sentencing disparity.” U.S.S.G. Ch. 6, Pt. B, intro, comment. To this end, a court may accept a plea agreement “if the court determines ... that accepting the agreement will not undermine the statutory purposes of sentencing or the sentencing guidelines.” U.S.S.G. § 6B1.2(a). Moreover, “a sentencing court may control any inappropriate manipulation of the indictment through use of its departure power.” U.S.S.G. Ch. 1, Pt. A, p.s. 4(a).

*746 Our conclusion that disparity in sentencing of § 1326 cases among federal districts may be a valid ground for downward departure is consistent with our decisions permitting departures in light of disparate sentencing schemes. See United States v. Ray, 930 F.2d 1368 (9th Cir.1990), as amended, Apr. 23, 1991 (upholding court’s authority to depart downward where defendant sentenced under Guidelines received more severe sentence than codefendants sentenced during period when Ninth Circuit rejected Guidelines), cert. denied, 498 U.S. 1124 , 111 S.Ct. 1084 , 112 L.Ed.2d 1189 (1991); United States v. Boshell, 952 F.2d 1101 (9th Cir.1991) (upholding downward departure where defendant’s criminal acts straddled the Guidelines’ effective date yet defendant charged only with Guideline crimes, while codefendants were given an opportunity to plead to pre-Guidelines crimes with lower sentences).

We are not persuaded by the Government’s argument that a downward departure is foreclosed by our decision in United States v. Mejia, 953 F.2d 461 (9th Cir.1991), as amended, Mar. 25, 1992, cert. denied, 504 U.S. 926 , 112 S.Ct. 1983 , 118 L.Ed.2d 581 (1992). In Mejia , we held that a district court cannot “depart downward for the purpose of avoiding unequal treatment of codefendants.” Id. at 468 . We explained:

Id. (citation omitted).

Mejia’s sentence achieved greater uniformity with other sentences generally, and departing to account for the disparity vis-a-vis his codefendant’s sentence would have created a greater disparity. In this case, by contrast, the disparity is between the Central District of California’s treatment of § 1326 defendants and the treatment accorded such defendants elsewhere in California. This greater disparity trumps whatever lesser uniformity there may be in how similarly situated defendants are treated within the Central District. Indeed, the differing § 1326 plea-bargaining policies of U.S. Attorneys in California result in a systemic sentencing disparity that is “ ‘highly infrequent’ ” among federal crimes. Koon, 518 U.S. at 96 , 116 S.Ct. at 2045 (quoting U.S.S.G. Ch. 1, Pt. A).

The validity of Mejia after the Supreme Court’s decision in Koon may be questionable. In Koon , the Court cautioned federal courts against categorically prohibiting factors as bases for departure when the factors are not discussed in the Sentencing Guidelines. 518 U.S. at 106-07 , 116 S.Ct. at 2050 . To the extent Mejia might be read to forbid consideration of sentencing disparity among co-defendants, such a categorical prohibition arguably is inconsistent with Koon. See United States v. Sanchez-Rodriguez, 161 F.3d 556, 560-61 (9th Cir.1998) (en banc) (“Koon made it clear that we cannot categorically forbid a district court from departing downward on any basis except for those specifically proscribed in the Guidelines.”) (footnote and citations omitted).

CONCLUSION

We are mindful of the Court’s admonition in Koon that departures based on grounds not mentioned in the Sentencing Guidelines will be “highly infrequent.” 518 U.S. at 96 , 116 S.Ct. at 2045 . Accordingly, we emphasize the narrowness of our decision. Differences in U.S. Attorneys’ plea-bargaining policies in California for § 1326 violations have apparently created gross sentencing disparities in neighboring federal districts. The Government has identified no other federal criminal statute that has spawned such enormous disparities. Because varying approaches to *747 § 1326 prosecution create this unique systemic disparity, our decision is necessarily limited to the unique circumstances of § 1326 prosecutions in the Central District of California.

We also emphasize that we do not today decide whether Banuelos-Rodriguez is actually entitled to a sentencing departure. It will be the task of the district court to make a “refined assessment of the many facts bearing on the outcome, informed by its vantage point and day-to-day experience in criminal sentencing.” Id. at 98, 116 S.Ct. at 2046-47 . We express no opinion whether the evidence Banuelos-Rodri-guez has presented is sufficient to warrant such a departure, nor do we speculate about what evidence would be necessary to merit a departure. Rather, we simply reiterate a rule made clear in Koon : it is the task of Congress, not the federal courts, to identify factors which are categorically forbidden as grounds for departure from the Sentencing Guidelines.

Gross sentencing disparities among similar contiguous federal districts because of different plea-bargaining policies of U.S. Attorneys in § 1326 prosecutions may be a valid ground for departure. The district court was incorrect when it held that this was not a proper factor to be considered for a departure. We remand so that the district court may exercise its discretion and determine whether the unique circumstances of Banuelos-Rodriguez’s case cause it to fall outside of the Sentencing Guidelines’ “heartland.”

REVERSED AND REMANDED.

. The Supreme Court held in Almendarez-Torres v. United States, 523 U.S. 224 , 118 S.Ct. 1219, 1226 , 140 L.Ed.2d 350 (1998), that 8 U.S.C. § 1326 (a) and § 1326(b) do not describe separate offenses. Rather, § 1326(a) describes the offense (illegal reentry), and § 1326(b) provides sentencing enhancements for that offense. See id. (clarifying relationship between § 1326(a) and § 1326(b)(2)); see also United States v. Alviso, 152 F.3d 1195 (9th Cir.1998) (§ 1326(a) and § 1326(b)(1)).