opinion: Mr. Justice Harlan
  delivered the opinion of the Court.
  Petitioner, alleging that it had been injured by respondent’s sales at unreasonably low prices in violation of § 3 of the Robinson-Patman Act, 49 Stat. 1526, 15 U. S. C. § 13a, sued the respondent for treble damages and injunc-tive relief under §§4 and 16 of the Clayton Act, 38 Stat. 730, as amended, 15 U. S. C. §§ 15, 26. The District Court dismissed the complaint on the ground that the private remedies afforded by §§ 4 and 16 of the Clayton Act cannot be based on a violation of § 3 of the Robinson-Patman Act. The Court of Appeals affirmed. 238 F. 2d 86. We brought the case here, 352 U. S. 1023, to resolve a conflict between the ruling below and a decision of the Court of Appeals for the Tenth Circuit holding that such a private action does lie. Vance v. Safeway Stores, Inc., 239 F. 2d 144.
  Sections 4 and 16 of the Clayton Act permit private actions of this kind only for injuries resulting from practices forbidden by the “antitrust laws” as defined in § 1 of the Clayton Act, namely: (1) the Sherman Act (Act of July 2, 1890); (2) parts of the Wilson Tariff Act (Act of August 27, 1894); (3) the Act amending the Wilson Tariff Act (Act of February 12, 1913); and (4) the Clayton Act (“this Act”). In light of the much other so-called antitrust legislation enacted prior and subsequent to the Clayton Act, it seems plain that the rule expressio unius exclusio alterius is applicable, and that the definition contained in § 1 of the Clayton Act is exclusive. Therefore it is of no moment here that the Robinson-Patman Act may be colloquially described as an “antitrust” statute. And since no one claims that § 3 of the Robinson-Patman Act can be regarded as an amendment to the Sherman Act or the Wilson Tariff Act, the precise issue before us is whether Congress made that section of the Robinson-Patman Act a part of the Clayton Act, thus making it one of the “antitrust laws” whose violation can lead to the private causes of action authorized by §§ 4 and 16. For the reasons stated below we hold that this is not the case.
  I.
  The Robinson-Patman Act, consisting of four sections, convincingly shows on its face that § 3 does not amend the Clayton Act, but stands on its own footing and carries its own sanctions.
  The first section of the Act does expressly amend § 2 of the Clayton Act, which prohibits certain kinds of price discriminations, and allied activities, on the part of those engaged in domestic or territorial commerce. The first paragraph of this section reads:
  “That section 2 of the [Clayton Act] ... is amended to read as follows:”
  The section then sets forth in haec verba, and within quotation marks, all the provisions of § 2, as modified by the amending language. 49 Stat. 1526, 15 U. S. C. § 13 (a).
  Two other sections of the Act are not in point here. Section 2 simply applies the amending provisions of § 1 to litigation commenced under the former provisions of § 2 of the Clayton Act, 15 U.. S. C. § 21a; and § 4 deals with certain practices of cooperative associations. 15 U. S. C. § 13b.
  The only other section of the Act is § 3, with which we are concerned here. It prohibits three kinds of trade practices, (a) general price discriminations, (b) geographical price discriminations, and (c) selling “at unreasonably low prices for the purpose of destroying competition or eliminating a competitor.” The important thing to note is that this section, in contrast to § 1 of the Robinson-Patman Act, does not on its face amend the Clayton Act. Further, § 3 contains only penal sanctions for violation of its provisions; in the absence of a clear expression of congressional intent to the contrary, these sanctions should under familiar principles be considered exclusive, rather than supplemented by civil sanctions of a distinct statute. See D. R. Wilder Mfg. Co. v. Corn Products Refining Co., 236 U. S. 165, 174-175.
  The conclusion that only § 1 of the Robinson-Patman Act can be regarded as amendatory of the Clayton Act is further borne out by the title of the whole Robinson-Patman Act, which reads (49 Stat. 1526):
  “An Act
  “To amend section 2 of [the Clayton Act] . . . and for other purposes.” (Italics added.)
  The “other purposes” can only refer to the sections of the Act other than the first section.
  Because there is a partial overlap between the price-discrimination clauses of § 3 of the Robinson-Patman Act (see note 1, supra) and those of § 2 of the Clayton Act, as amended by the first section of the Robinson-Patman Act, it is argued that it would be anomalous to allow a private cause of action for price discrimination in violation of § 2 of the Clayton Act but to deny a private cause of action based on a violation of § 3 of the Robinson-Patman Act. This argument, however, overlooks the faet that § 3 of the Robinson-Patman Act includes a provision which is not found in § 2 of the Clayton Act, namely, selling “at unreasonably low prices for the purpose of destroying competition or eliminating a competitor.” It is not an idle conjecture that the possibility of abuse inherent in a private cause of action based upon this vague provision was among the factors which led Congress to leave the enforcement of the provisions of § 3 solely in the hands of the public authorities, except to the extent that violation of any of its provisions also constituted a violation of § 2 of the Clayton Act, and as such was subject to private redress under §§ 4 and 16 of that Act. In any event, in the absence of a much clearer indication of congressional intent than is present in these statutory provisions and their legislative history (infra, p. 380), we should not read the Robinson-Patman Act as subjecting violations of the “unreasonably low prices” provision of § 3 to the private remedies given by the Clayton Act.
  Respondent calls our attention to the fact that the 1940 U. S. Code codifies § 3 of the Robinson-Patman Act as being among the “antitrust laws” embraced in § 1 of the Clayton Act. However, reference to the 1926 and 1934 Codes shows that the 1940 codification was a palpable error. Moreover, this codification seems to us, for the reasons set forth in this opinion, to be manifestly inconsistent with the Robinson-Patman Act, and in such circumstances Congress has specifically provided that the underlying statute must prevail. Act of June 30, 1926, § 2 (a), vol. 1 U. S. C. (1952 ed.), p. Lxiii; see Stephan v. United States, 319 U. S. 423, 426.
  II.
  What appears from the face of the Robinson-Patman Act finds full support in its legislative history. The fair conclusions to be drawn from that history are (a) that § 3 of the Robinson-Patman Act was not intended to become part of the Clayton Act, and (b) that the section was intended to carry only criminal sanctions, except that price discriminations, to the extent that they were common to both that section and § 2 of the Clayton Act, were also understood to carry, under the independent force of the Clayton Act, the private remedies provided in §§ 4 and 16 of the Clayton Act. In other words, although price discriminations are both criminally punishable (under § 3 of the Robinson-Patman Act) and subject to civil redress (under § 2 of the Clayton Act), selling “at unreasonably low prices” is subject only to the criminal penalties provided in § 3 of the Robinson-Patman Act. This is evident from the Conference Report on the bill, which states:
  “section 2
  “The provisions of section 2 of the House bill were agreed to without amendment by the Senate. . . . [I]t appears in the conference report as section 2 of the bill itself, rather than as part of the amendment to section 2 of the Clayton Act which is provided for in section 1 to the present bill.
  “section 3
  “Subsection (h) of the Senate amendment . . . appears in the conference report as section 3 of the bill itself. It contains the operative and penal provisions of what was originally the Borah-Van Nuys bill (S. 4171) , While they overlap in some respects, they are in no way inconsistent with the provisions of the Clayton Act amendment provided for in section 1. Section 3 authorizes nothing which that amendment prohibits, and takes nothing from it. On the contrary, where only civil remedies and liabilities attach to violations of the amendment provided in section 1, section 3 sets up special prohibitions as to the particular offenses therein described and attaches to them also the criminal penalties therein provided.” H. R. Rep. No. 2951, 74th Cong., 2d Sess., p. 8. (Italics added.)
  Further excerpts from the legislative history, set forth in the margin, also bear out the conclusions stated at the outset of this part of our opinion.
  Finally, it is noteworthy, by way of epitomizing the conclusions to be drawn from the legislative history, that in 1950 Representative Patman (a coauthor of the Robinson-Patman Act) stated in testimony before a Subcommittee of the House Committee on the Judiciary (Hearing on H. R. 7905, 81st Cong., 2d Sess., Serial No. 14, Part 5, p. 48):
  “. . . it happens that section 3, the criminal section of the Robinson-Patman Act, was not, under the terms of that act, made an amendment to the Clayton Act. Moreover, section 3 of the Robinson-Patman Act has never been added to the list of laws designated as 'antitrust laws’ in section 1 of the Clayton Act.”
  For the foregoing reasons, we hold that a private cause of action does not lie for practices forbidden only by § 3 of the Robinson-Patman Act. To the extent that such practices also constitute a violation of § 2 of the Clayton Act, as amended, they are actionable by one injured thereby solely under that Act. Since no such infringement of § 2 is alleged here, the complaint in this case was properly dismissed.
  Affirmed.
  Section 3 of the Robinson-Patman Act provides:
  “It shall be unlawful for any person engaged in commerce, in the course of such commerce, to be a party to, or assist in, any transaction of sale, or contract to sell, which discriminates to his knowledge against competitors of the purchaser, in that, any discount, rebate, allowance, or advertising service charge is granted to the purchaser over and above any discount, rebate, allowance, or advertising service charge available at the time of such transaction to said competitors in respect of a sale of goods of like grade, quality, and quantity; to sell, or contract to sell, goods in any part of the United States at prices lower than those exacted by said person elsewhere in the United States for the purpose of destroying competition, or eliminating a competitor in such part of the United States; or, to sell, or contract to sell, goods at unreasonably low prices for the purpose of destroying competition or eliminating a competitor.
  “Any person violating any of the provisions of this section shall, upon conviction thereof, be fined not more than $5,000 or imprisoned not more than one year, or both.”
  Section 4 of the Clayton Act provides:
  “That any person who shall be injured in his business or property by reason of anything forbidden in the antitrust laws may sue therefor in any district court of the United States in the district in which the defendant resides or is found or has an agent, without respect to the amount in controversy, and shall recover threefold the damages by him sustained, and the cost of suit, including a reasonable attorney’s fee.”
  Section 16 of the Clayton Act grants a private cause of action for injunctive relief against “threatened loss or damage by a violation of the antitrust laws.”
  38 Stat. 730. Section 1 of the Clayton Act provides:
  “That ‘antitrust laws,’ as used herein, includes the Act entitled ‘An Act to protect trade and commerce against unlawful restraints and monopolies,’ approved July second, eighteen hundred and ninety; sections seventy-three to seventy-seven, inclusive, of an Act entitled ‘An Act to reduce taxation, to provide revenue for the Government, and for other purposes,’ of August twenty-seventh, eighteen hundred and ninety-four; an Act entitled ‘An Act to amend sections seventy-three and seventy-six of the Act of August twenty-seventh, eighteen hundred and ninety-four, entitled “An Act to reduce taxation, to provide revenue for the Government, and for other purposes,” ’ approved February twelfth, nineteen hundred and thirteen; and also this Act.”
  A total of 71 statutes (including the Clayton Act) are set forth in a compilation prepared by Elmer A. Lewis, Superintendent of the Document Room, House of Representatives, entitled Antitrust Laws with Amendments, 1890-1951 (1951). Of these statutes, 21 were on the books in 1914 when the Clayton Act was enacted, and 49 became law thereafter.
  The issue now before us was not decided in Bruce’s Juices, Inc., v. American Can Co., 330 U. S. 743, or Moore v. Mead’s Fine Bread Co., 348 U. S. 115.
  15 U. S. C. § 13 (a). Section 2 of the Clayton Act, as amended, provides:
  “It shall be unlawful for any person engaged in commerce, in the course of such commerce, either directly or indirectly, to discriminate in price between different purchasers of commodities of like grade and quality . . . where the effect of such discrimination may be substantially to lessen competition or tend to create a monopoly in any line of commerce, or to injure, destroy, or prevent competition with any person who either grants or knowingly receives the benefit of such discrimination, or with customers of either of them . . .
  We need not decide whether violations of the price discrimination provisions of § 3 of the Robinson-Patman Act are subject to all of the defenses provided in the case of price discriminations under the Clayton Act.
  The District Court indicated that the vagueness of the “unreasonably low prices” provision might give rise to constitutional difficulties, if such questions had to be faced. Cf. United, States v. L. Cohen Grocery Co., 255 U. S. 81; Cline v. Frink Dairy Co., 274 U. S. 445. See Comment, 55 Mich. L. Rev. 845, 853-856. Be that as it may, it is worthy of note that the Department of Justice has never, so far as we have been able to determine, brought proceedings under this provision of § 3.
  In the 1926 U. S. Code, § 1 of the Clayton Act was codified in part as follows (15 U. S. C. § 12):
  “ ‘Antitrust laws,’ as used in sections 12 to 27, inclusive, of this chapter [the Clayton Act], includes sections 1 to 27, inclusive, of this chapter.”
  This codification was correct because §§ 1-27 of Title 15 were the Sherman Act, the Wilson Tariff Act (as amended) and the Clayton Act. The 1934 Code was identical and also correct.
  The error occurred in the 1940 codification. The Robinson-Patman Act was enacted in 1936. In the 1940 Code the codification of § 1 of the Clayton Act was changed so that it read:
  “ ‘Antitrust laws,’ as used in sections 12, 13, 14-21, 22-27 of this title, includes sections 1-27 inclusive, of this title.”
  Sections 2, 3, and 4 of the Robinson-Patman Act had been codified as 15 U. S. C. §§ 21a, 13a and 13b, respectively. The codifiers partially recognized that these sections were not part of the Clayton Act by changing the figures “12 to 27” in the earlier codifications of 15 U. S. C. § 12 to read “12, 13, 14-21, 22-27.” But the codifiers failed to make a corresponding change in the figures “1 to 27” appearing in the earlier codifications. The result is that the term “antitrust laws” as used in § 1 of the Clayton Act appears in the 1940 Code to include § 3 of the Robinson-Patman Act, codified as § 13a. The 1946 and 1952 codifications perpetuated this error.
  Read in context, the legislative excerpts quoted in the dissenting opinion indicate no more than that.
  The House bill was introduced by Representative Patman. H. R. 8442, 79 Cong.' Re.c. 9081. Shortly thereafter an identical bill was introduced in the Senate by Senator Robinson. S. 3154, 79 Cong. Rec. 10129.
  Independently of the Robinson bill, Senators Borah and Van Nuys introduced separate price-discrimination bills. S. 3670, 80 Cong. Rec. 461; S. 3835, 80 Cong. Rec. 1194. These bills were later consolidated, S. 4171, 80 Cong. Rec. 3204, and ultimately the consolidated bill became § 3 of the Robinson-Patman Act.
  Representative Utterback, senior House Manager of the committee of conference, stated on the floor of the House (80 Cong. Rec. 9419):
  “Section 3 of the bill sets aside certain practices therein described and attaches to their commission the criminal penalties of fine and imprisonment therein provided. It does not affect the scope or operation of the prohibitions or limitations laid down by the Clayton Act amendment provided for in section 1. It authorizes nothing therein prohibited. It detracts nothing from them. Most of the acts which it does prohibit lie also within the prohibitions of that amendment. In that sphere this section merely attaches to them its criminal penalties in addition to the civil liabilities and remedies already provided by the Clayton Act.” (Italics added.)
  Representative Miller, a House Manager of the committee of conference, later said "Section 3 is the Borah-Van Nuys amendment. . . . The first section of the bill as reported back here amends section 2 of the Clayton Act.” When asked whether § 3 was “a part of the same act,” Mr. Miller replied (80 Cong. Rec. 9421):
  “Of course it is, but it is not a part of the Clayton Act as amended by section 2 [Section 1 of the Robinson-Patman Bill].” (Italics added.)
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