MEMORANDUM *

Petitioners appeal the district court’s denial of their motion to amend their pleadings pursuant to Fed.R.Civ.P. 15, and subsequent order dismissing their habeas corpus petitions. We reverse. Because the parties are well familiar with the long procedural history of this case, we will not recount it here.

I

The district court erred in denying petitioners’ motions to amend. We review the district court’s refusal to grant leave to amend under Rule 15 for abuse of discretion. Griggs v. Pace Am. Group, Inc., 170 F.3d 877 , 879 (9th Cir.1999). We review de novo the underlying legal conclusion of whether a particular amendment to the complaint would be futile. Griggs, 170 F.3d at 879; Steckman v. Hart Brewing, Inc., 143 F.3d 1293, 1296 (9th Cir.1998).

Fed.R.Civ.P. 15(b) allows the petitioners to amend their pleadings to conform to the evidence “at any time, even after judgment.” Here, the petitioners asserted that they were eligible for early release because the procedure by which the Bureau of Prisons (“Bureau”) rules were enacted was flawed. The initial petitions from John Gavis and Conrad Niemeier, for instance, alleged that “the new rules were neither properly promulgated nor authorized by law.” Further, the petitioners’ consolidated memorandum in sup *439 port of the habeas corpus petition detailed a claim based on the Administrative Procedure Act (“APA”), 5 U.S.C. § 553 , as an alternative ground for relief. 1 Therefore, the petitioners should have been permitted to amend them pleadings to include a claim under the APA.

Rule 15(b)’s requirement that the petitioners establish that the parties exhibited “express or implied consent” is also no barrier to the petitioners’ motions for amendment. The BOP exhibited implied consent as to the APA claim. In response to the Gavis and Niemeier petitions, the BOP responded that the interim rule was “properly promulgated” and therefore “entitled to full deference.” Likewise, the BOP’s answer to the petitioners’ consolidated memorandum asserted that the “new interim rule” was “properly promulgated,” and was therefore “entitled to full deference.”

For these reasons, we conclude that the district court abused its discretion by prohibiting the petitioners from amending their pleadings under Rule 15(b) to conform to the evidence.

II

Contrary to the district court’s conclusion, such an amendment would not have been futile. The petitioners claim that 1997 regulation was promulgated in violation of the APA. A regulation is invalid if the agency fails to follow procedures required by the APA. Buschmann v. Schweiker, 676 F.2d 352, 355-56 (9th Cir. 1982). The APA requires that an agency (1) publish notice of a proposed rule in the Federal Register not less than thirty days before the rule’s effective date, and (2) provide an opportunity for the public to comment on the rule. 5 U.S.C. § 553 . 2 The APA applies to substantive rules (which “impose new rights or obligations by changing an existing law”) but not to interpretive rules (which “clarify or explain existing law or regulations so as to advise the public of the agency’s construction of the rules it administers”). Gunderson v. Hood, 268 F.3d 1149, 1154 (9th Cir.2001); see also 5 U.S.C. § 553 (b)(3)(A). The APA also makes exceptions to its advance notice requirement for “a substantive rule which grants or recognizes an exemption or relieves a restriction” and “as otherwise provided by the agency for good cause found and published with the rule.” 5 U.S.C § 553(d)(1) & (3). 3 Whether an agency pronouncement is interpretive or substantive (and therefore generally subject to the *440 APA) is a legal question that is reviewed de novo. Gunderson, 268 F.3d at 1153-54 .

Gunderson settled the issue of whether the 1997 regulation is substantive; it clearly is. Id. at 1154 (“There is no question that the 1997 regulation is substantive.” [emphasis in original]). Based on the undisputed facts as to the procedural history of the rule promulgation, it is apparent that the procedures utilized to adopt the regulation did not comply with the APA. The Bureau violated the APA’s thirty-day advance notice requirement by making the 1997 regulation effective as of October 9, 1997, rather than as of November 14, 1997, thirty days after the October 15, 1997 publication in the Federal Register. Thus, based on the undisputed facts, the requirements of the APA were not observed.

The BOP does not dispute this, but argues that statutory exceptions apply. Determination of the applicability of these exceptions requires further development of the record; thus, we cannot say that the asserted statutory exceptions preclude petitioners’ APA claims as a matter of law without the necessity of any further factual development.

The district court also incorrectly concluded that the proposed amendment would be futile because the appellants have not exhausted their administrative remedies. In reaching that conclusion, the district court relied on Booth v. Churner, 532 U.S. 731 , 121 S.Ct. 1819 , 149 L.Ed.2d 958 (2001), which held that exhaustion is required under the Prison Litigation Reform Act (“PLRA”), 42 U.S.C.1997e(a), even where a prisoner litigant seeks a remedy that the available administrative process does not provide, as long as the agency has the authority to take some responsive action. However, Booth is inapposite. Congress has made no indication that habeas petitioners are statutorily required to exhaust administrative remedies before filing under Section 2241 for habeas corpus relief. See Brown v. Rison, 895 F.2d 533, 535 (9th Cir.1990) (“The requirement that federal prisoners exhaust remedies before filing a habeas corpus petition was judicially created; it is not a statutory requirement.”).

Ill

In sum, it would not have been futile to allow petitioners to amend their complaints, and the district court should have permitted the amendments pursuant to Fed.R.Civ.P. 15(b). Therefore, reversal is required to permit the amendments. We do not opine on any other issue urged by the parties.

REVERSED.

This disposition is not appropriate for publication and may not be cited to or by the courts of this circuit except as may be provided by Ninth Circuit Rule 36-3.

. The memorandum asserts that "[t]he October 1997 rule disqualifying the prisoners from Section 3621(e) early release eligibility are rules requiring prospective notice and comment [and] the [Bureau] was required to follow the publication rules provided under the APA." Moreover, in regard to the 1997 program statement, the memorandum claims that "the new program statement is void because it was promulgated in violation of the Administrative Procedure Act.”

. An improperly promulgated interim rule is invalid as to persons disqualified prior to the issuance of the final rule. See Buschmann v. Schweiker, 676 F.2d 352, 358 (9th Cir. 1982); see also Bowen v. Georgetown Univ. Hosp., 488 U.S. 204, 225 , 109 S.Ct. 468 , 102 L.Ed.2d 493 (1988) (Scalia, J., concurring) ("acceptance of the Secretary’s position would make a mockery ... of the APA, since agencies would be free to violate the rulemaking requirements of the APA with impunity if, upon invalidation of a rule, they were free to reissue that rule on a retroactive basis.”) (internal quotation marks omitted, ellipsis in original).

. To be precise, this section provides that ”[t]he required publication or service of a substantive rule shall be made not less than 30 days before its effective date, except — (1) a substantive rule which grants or recognizes an exemption or relieves a restriction; ... or (3) as otherwise provided by the agency for good cause found and published with the rule.” 5 U.S.C. § 553 (d)(1) & (3).