delivered the opinion of the Court.
Section 30 of the National Bank Act of 1864, Rev. Stat. § 5197, as amended, 12 U. S. C. § 85, provides that a national bank may charge its loan customers “interest at the rate allowed by the laws of the State... where the bank is located.” In
Marquette Nat. Bank of Minneapolis
v.
First of Omaha Service Corp.,
I
Petitioner, a resident of California, held two credit cards— a “Classic Card” and a “Preferred Card”—issued by respond *738 ent, a national bank located in Sioux Falls, South Dakota. The Classic Card agreement provided that respondent would charge petitioner a late fee of $15 for each monthly period in which she failed to make her minimum monthly payment within 25 days of the due date. Under the Preferred Card agreement, respondent would impose a late fee of $6 if the minimum monthly payment was not received within 15 days of its due date; and an additional charge of $15 or 0.65% of the outstanding balance on the Preferred Card, whichever was greater, if the minimum payment was not received by the next minimum monthly payment due date. Petitioner was charged late fees on both cards.
These late fees are permitted by South Dakota law, see S. D. Codified Laws §§54-3-1, 54-3-1.1 (1990 and Supp. 1995). Petitioner, however, is of the view that exacting such “unconscionable” late charges from California residents violates California law, and in 1992 brought a class action against respondent on behalf of herself and other California holders of respondent’s credit cards, asserting various statutory and common-law claims.
1
Respondent moved for judgment on the pleadings, contending that petitioner’s claims were pre-empted by §85. The Superior Court of Los Angeles County initially denied respondent’s motion, but the California Court of Appeal, Second Appellate District, issued a writ of mandate directing the Superior Court to either grant the motion or show cause why it should not be required to do so. The Superior Court chose the former course, and the Court of Appeal affirmed its dismissal of the complaint,
II
In light of the two dissents from the opinion of the Supreme Court of California, see
On March 3,1995, which was after the California Superior Court’s dismissal of petitioner’s complaint, the Comptroller of the Currency noticed for public comment a proposed regu *740 lation dealing with the subject before us, see 60 Fed. Reg. 11924, 11940, and on February 9, 1996, which was after the California Supreme Court’s decision, he adopted the following provision:
“The term ‘interest’ as used in 12 U. S. C. § 85 includes any payment compensating a creditor or prospective creditor for an extension of credit, making available of a line of credit, or any default or breach by a borrower of a condition upon which credit was extended. It includes, among other things, the following fees connected with credit extension or availability: numerical periodic rates, late fees, not sufficient funds (NSF) fees, over limit fees, annual fees, cash advance fees, and membership fees. It does not ordinarily include appraisal fees, premiums and commissions attributable to insurance guaranteeing repayment of any extension of credit, finders’ fees, fees for document preparation or notarization, or fees incurred to obtain credit reports.” 61 Fed. Reg. 4869 (to be codified in 12 CFR § 7.4001(a)).
Petitioner proposes several reasons why the ordinary rule of deference should not apply to this regulation. First, petitioner points to the fact that this regulation was issued more than 100 years after the enactment of § 85, and seemingly as a result of this and similar litigation in which the Comptroller has participated as
amicus curia#
on the side of the banks. The 100-year delay makes no difference. To be sure, agency interpretations that are of long standing come before us with a certain credential of reasonableness, since it is rare that error would long persist. But neither antiquity nor contemporaneity with the statute is a condition of validity. We accord deference to agencies under
Chevron,
not because of a presumption that they drafted the provisions in question, or were present at the hearings, or spoke to the principal sponsors; but rather because of a presumption that Congress, when it left ambiguity in a statute meant
*741
for implementation by an agency, understood that the ambiguity would be resolved, first and foremost, by the agency, and desired the agency (rather than the courts) to possess whatever degree of discretion the ambiguity allows. See
Chevron, supra,
at 843-844. Nor does it matter that the regulation was prompted by litigation, including this very suit. Of course we deny deference “to agency litigating positions that are wholly unsupported by regulations, rulings, or administrative practice,”
Bowen
v.
Georgetown Univ. Hospital,
Second, petitioner contends that the Comptroller’s regulation is not deserving of our deference because “there is no rational basis for distinguishing the various charges [it] has denominated interest. . . from those charges it has denominated ‘non-interest.’” Reply Brief for Petitioner 14. We disagree. As an analytical matter, it seems to us perfectly possible to draw a line, as the regulation does, between (1) “payment compensating a creditor or prospective creditor for an extension of credit, making available of a line of credit, or any default or breach by a borrower of a condition upon which credit was extended,” and (2) all other payments. To be sure, in the broadest sense
all
payments connected in any way with the loan — including reimbursement of the lender’s costs in processing the application, insuring the loan, and appraising the collateral — can be regarded as “compensating [the] creditor for [the] extension of credit.” But it seems to us quite possible and rational to distinguish, as the regulation does, between those charges that are
specifically as
*742
signed,
to such expenses and those that are assessed for simply making the loan, or for the borrower’s default. In its logic, at least, the line is not “arbitrary [or] capricious,” and thereby disentitled to deference under
Chevron,
see
Finally, petitioner argues that the regulation is not entitled to deference- because it is inconsistent with positions taken by the Comptroller in the past. Of course the mere fact that an agency interpretation contradicts a prior agency position is not fatal. Sudden and unexplained change, see,
e. g., Motor Vehicle Mfrs. Assn. of United States, Inc.
v.
State Farm Mut. Automobile Ins. Co.,
In any case, we do not think that anything which can accurately be described as a change of official agency position has occurred here. The agency’s Notice of Proposed Rule-making asserted that the new regulation “reflects] current law and [Office of the Comptroller of the Currency (OCC)] interpretive letters,” 60 Fed. Reg. 11929 (1995), and the Statement of Basis and Purpose accompanying the final adoption stated that “[t]he final ruling is consistent with OCC interpretive letters in this area . . . and reflects the position the OCC has taken in amicus curiae briefs in litigation pending in many state and Federal courts,” 61 Fed. Reg. *743 4859 (1996) (citing OCC interpretive letters). Petitioner points only to (1) a June 1964 letter from the Comptroller to the President’s Committee on Consumer Interests, which states that “[c]harges for late payments, credit life insurance, recording fees, documentary stamp are illustrations of charges which are made by some banks which would not properly be characterized as interest,” see App. to Brief for Petitioner 5a; and (2) a 1988 opinion letter from the Deputy Chief Counsel of the OCC stating “it is my position that [under §85] the laws of the states where the banks are located . . . determine whether or not the banks can impose the foregoing fees and charges [including late fees] on Iowa residents,” OCC Interpretive Letter No. 452, reprinted in 1988-1989 Transfer Binder, CCH Fed. Banking L. Rep. ¶ 85,676, p. 78,064 (1988). We doubt whether either of these statements was sufficient in and of itself to establish a binding agency policy — the former, because it was too informal, and the latter because it only purported to represent the position of the Deputy Chief Counsel in response to an inquiry concerning particular banks. Nor can it even be argued that the two statements reflect a prior agency policy, since, in addition to contradicting the regulation before us here, they also contradict one another — the former asserting that “interest” is a nationally uniform concept, and the latter that it is to be determined by reference to state law. What these statements show, if anything, is that there was good reason for the Comptroller to promulgate the new regulation, in order to eliminate uncertainty and confusion.
In addition to offering these reasons why 12 CFR § 7.4001(a) in particular is not entitled to deference, petitioner contends that
no
Comptroller interpretation of § 85 is entitled to deference, because §85 is a provision that preempts state law. She argues that the “presumption against . . . pre-emption” announced in
Cipollone
v.
Liggett Group, Inc.,
HH HH HH
Since we have concluded that the Comptrollers regulation deserves deference, the question before us is not whether it represents the best interpretation of the statute, but *745 whether it represents a reasonable one. The answer is obviously yes.
Petitioner argues that the late fees charged by respondent do. not constitute “interest” because they “do not vary based on the payment owed or the time period of delay.” Brief for Petitioner 32-33. We do not think that such a limitation must be read into the statutory term. Most legal dictionaries of the era of the National Bank Act did not place such a limitation upon “interest.” See,
e. g.,
1 J. Bouvier, A Law Dictionary 652 (6th ed. 1856) (“The compensation which is paid by the borrower to the lender or by the debtor to the creditor for... use [of money]”); 2 A. Burrill, A Law Dictionary and Glossary 90 (2d ed. 1860); 11 American and English Encyclopedia of Law 379 (J. Merrill ed. 1890). But see J. Wharton, Law Lexicon or Dictionary of Jurisprudence 391 (2d Am. ed. 1860). The definition of “interest” that we ourselves set out in
Brown
v.
Hiatts,
Petitioner suggests another source for the asserted requirement that the charges be time- and rate-based: What is authorized by § 85, she notes, is the charging of interest “at the rate allowed” by the laws of the bank’s home State. This requires, in her view, that the interest charges be expressed as functions of time and amount owing. It would be surprising to find such a requirement in the Act, if only because it would be so pointless. Any flat charge may, of course, readily be converted to a percentage charge — which
*746
was indeed the basis for 19th-century decisions holding that flat charges violated state usury laws establishing maximum “rates.” See,
e. g., Craig
v.
Pleiss,
Finally, petitioner contends that the late fees cannot be “interest” because they are “penalties.” To support that dichotomy, she points to our opinion in
Meilink
v.
Unemployment Reserves Comm’n of Cal.,
* * *
Petitioner devotes much of her brief to the question whether the meaning of “interest” in § 85 can constitutionally be left to be defined by the law of the bank’s home State — a question that is not implicated by the Comptroller’s regulation. Because the regulation is entitled to deference, and because the Comptroller’s interpretation of §85 is not an unreasonable one, the decision of the Supreme Court of California must be affirmed.
It is so ordered.
Notes
By way of common-law claims, petitioner’s complaint alleged breach of duty of good faith and fair dealing; unjust enrichment; fraud and deceit; negligent misrepresentation; and breach of contract. It also alleged violation of Cal. Bus. & Prof. Code Ann. § 17200 (West Supp. 1996) (prohibiting unlawful business practices) and Cal. Civ. Code Ann. §1671 (West 1985) (invalidating unreasonable liquidated damages).
Sherman
v.
Citibank (South Dakota), N.
A., 143 N. J. 35,
In a four-line footnote on the last page of her reply brief, and unpur-sued in oral argument, petitioner raised the point that deferring to the regulation in this case involving antecedent transactions would make the regulation retroactive, in violation of
Bowen
v.
Georgetown Univ. Hospital,