delivered the opinion of the Court.

This is a denaturalization proceeding under § 340 (a) of the Immigration and Nationality Act of 1952.1 The sole question is whether § 340 (a) makes the filing of the “affidavit showing good cause” a prerequisite to maintenance of the suit. The District Court held that it does *92and ordered the complaint dismissed unless the Government filed an affidavit showing good cause within 60 days. As this was not done, the complaint was dismissed without prejudice to the Government’s right to institute an action to denaturalize the respondent upon filing the affidavit. 125 F. Supp. 551. On appeal by the Government the Court of Appeals for the Second Circuit affirmed, adopting the opinion of the District Court. 221 F. 2d 805. We granted certiorari, 350 U. S. 817, because of an asserted conflict with decisions of the Seventh2 and Ninth3 Circuits and because of the importance of the question in the administration of the immigration and naturalization laws.

Respondent Ettore Zueca was naturalized on January 4, 1944. In 1954, the United States Attorney for the Southern District of New York, proceeding under §340 (a), filed a verified complaint in the United States District Court in his District seeking revocation of respondent’s naturalization on the grounds of illegality, concealment of material facts, and willful misrepresentation.

The complaint alleged that respondent, at his naturalization hearing and in his petition for naturalization, had falsely sworn “that he did not belong to and was not associated with any organization which teaches or advocates the overthrow of existing government in this country . . . ,” that it was his “intention in good faith to become a citizen of the United States and to renounce absolutely and forever all allegiance and fidelity to any [foreign powers] . . . ,” and that he was and had been “attached to the principles of the Constitution of the United States and well disposed to the good order and happiness of the United States.” This was followed by *93a general allegation of membership in the Communist Party and “other organizations affiliated with or controlled by the Communist Party of the United States from 1925 to 1947,” and equally general allegations that respondent procured his naturalization by concealment and willful misrepresentation in that he concealed the facts relating to his membership in the Communist Party and affiliated organizations and otherwise swore falsely as to his intentions and beliefs. The pleader concluded that “good cause exists for the institution of this suit . . . .” The complaint, no part of which was alleged on information and belief, was verified by an Assistant United States Attorney. When respondent sought to take depositions of this Attorney, he was met with an affidavit in opposition denying personal knowledge.4 Respondent then filed his motion to dismiss on the ground, inter alia, that § 340 (a) required the filing of an affidavit showing good cause and that this requirement had not been complied *94with. As stated above, the motion to dismiss was granted on this ground.

The Government argues that a reading of the statute and its legislative history leads to the conclusion that the filing of an “affidavit showing good cause” is not a prerequisite to maintaining denaturalization proceedings under § 340 (a). We do not agree.

The affidavit provision with which we are here concerned first appeared in § 15 of the Act of June 29, 1906.5 Without substantial change, it was carried forward in the laws of 1940 6 and 1952,7 currently reading as follows:

“Sec. 340. (a) It shall be the duty of the United States district attorneys for the respective districts, upon affidavit showing good cause therefor, to institute proceedings in any court specified in subsection (a) of section 310 of this title in the judicial district in which the naturalized citizen may reside at the time of bringing suit, for the purpose of revoking and setting aside the order admitting such person to citizenship and canceling the certificate of naturalization on the ground that such order and certificate of naturalization were procured by concealment of a material fact or by willful misrepresentation, . . . .” (Emphasis added.)

Were we obliged to rely solely on the wording of the statute, we would have no difficulty in reaching the conclusion that the filing of the affidavit is a prerequisite to maintaining a denaturalization suit. This conclusion is not altered by a consideration of the Government’s highly *95speculative suggestions as to the meaning of the legislative history. On the contrary, we think that it is entirely consistent with the Court’s statement in Bindczyck v. Finucane, 342 U. S. 76, that Congress acted “[w]ith a view to protecting the Government against fraud while safeguarding citizenship from abrogation except by a clearly defined procedure . . . .” Id., at 83.

The natural meaning of the language used in § 340 (a) is that filing of the affidavit is a procedural prerequisite to maintenance of the suit. In the Bindczyck case, this Court held that § 338 (a) of the Nationality Act of 1940, predecessor of § 340 (a), sets forth the exclusive procedure for denaturalization.8 Despite that decision, the Government would have us hold now that the grant of power to maintain denaturalization suits is found in the general duty of United States Attorneys to prosecute all civil actions in which the United States is concerned,9 and that § 340 (a) merely imposes the “additional duty ... to act, not alone on their own knowledge and judgment, but on the basis of an affidavit of good cause furnished by private citizens.” In effect the Government argues that the affidavit is required only when the proceeding is to be brought on the complaint of a private citizen. We need not decide whether a private citizen may ever file such a complaint. The short answer in this case is that the Government laid its complaint expressly under § 340 (a).

While arguing that the words of § 340 are words of limitation on the discretion of the United States Attorney, the Government apparently concedes that the *96venue -and notice provisions of the Section are generally applicable to denaturalization proceedings. Its argument overlooks the fact that the affidavit and venue provisions are in the same sentence. If the affidavit were required only when the United States Attorney proceeded on the complaint of a private citizen, then only in such a case would the venue be restricted to the district of the defendant's residence. We could accept such a limiting construction of the statute only upon a very clear showing that Congress meant something other than what it said.

The original Act of 1906 was the culmination of many years of study by Congress and a commission of which the Attorney General was a leading spirit and his Assistant the Chairman. Shortly after its enactment, the same Attorney General rendered an opinion to the Secretary of Commerce and Labor to the effect that the filing of an affidavit was “necessary to give a United States attorney authority to institute proceedings in any court for the cancellation of a naturalization certificate.” 10 In such circumstances, a contemporaneous construction of a statute by the officer charged with its enforcement is entitled to great weight. Fawcus Machine Co. v. United States, 282 U. S. 375, 378; Norwegian Nitrogen Products Co. v. United States, 288 U. S. 294, 315. Over a period of years, however, there has been some conflict among the circuits concerning the affidavit requirement.11 The Government *97relies particularly on our affirmance of the decision of the Court of Appeals for the Ninth Circuit in Schwinn v. United States, 311 U. S. 616. The Court decided that case on the sole ground of illegal procurement. Although the necessity of the affidavit was considered below in view of the Government’s failure to offer proof in support of its allegation that an affidavit of good cause had been furnished, the question was not presented to this Court.12

Prior to the decision in the Schwinn case the practice of the Justice Department seems generally to have been to institute denaturalization proceedings upon affidavit showing good cause. The Government does not now contend that it has abandoned that practice. It merely claims the right not to do so when it chooses, as clearly appears from portions of a Department of Justice memorandum which has been brought to the Court’s attention.13 *98We are unimpressed by the reasoning of that memorandum. We think that the public interest is not served by taking such liberties with a specific statutory requirement designed for the protection of naturalized citizens. And we fail to see that the requirement imposes a burden on the Government. At this Term, it has been represented to us that the usual practice is that, “if sufficient grounds are shown, an ‘affidavit showing good cause’ (see Section 340 (a) of the Act . . .) is prepared and executed, and is forwarded as an aid to the formal judicial proceedings . . . .”14

Lastly the Government contends, as an alternate ground for reversal, that no harm is done to the defendant because the complaint itself is verified and, therefore, accomplishes the function of the affidavit. With this we cannot agree. The complaint, under modern practice, is required merely *99to allege ultimate facts while the affidavit must set forth evidentiary matters showing good cause for cancellation of citizenship.15

In the Bindczyck case, supra, the Court summed up the purpose of and approach to denaturalization proceedings as follows:

“[W]e cannot escape the conclusion that in its detailed provisions for revoking a naturalization because of fraud or illegal procurement not appearing on the face of the record, Congress formulated a self-contained, exclusive procedure. With a view to protecting the Government against fraud while safeguarding citizenship from abrogation except by a clearly defined procedure, Congress insisted on the detailed, explicit provisions of § 15.

. . if citizenship was granted, it was to be proof against attacks for fraud or illegal procurement based on evidence outside the record, except through the proceedings prescribed in § 15. The congressional scheme, providing carefully for the representation of the Government’s interest before the grant of citizenship and a detailed, safeguarded procedure for attacking the decree on evidence of fraud outside the record, covers the whole ground. Every national interest is thereby protected.” 342 U. S., at 83, 84.

This proceeding was concededly brought under § 340 (a). We hold that this is the only Section under which a United States attorney may institute denaturali-zation proceedings, and that the affidavit showing good cause is a procedural prerequisite to the maintenance of proceedings thereunder.

The mere filing of a proceeding for denaturalization results in serious consequences to a defendant. Even if *100his citizenship is not cancelled, his reputation is tarnished and his standing in the community damaged. Congress recognized this danger and provided that a person, once admitted to American citizenship, should not be subject to legal proceedings to defend his citizenship without a preliminary showing of good cause. Such a safeguard must not be lightly regarded. We believe that, not only in some cases but in all cases, the District Attorney must, as a prerequisite to the initiation of such proceedings, file an affidavit showing good cause. The District Court below correctly dismissed the proceedings in this case because of the failure of the Government to file the required affidavit and the judgment of the Court of Appeals is therefore

A „ , Affirmed.

dissenting.

The Court’s ruling today seriously obstructs the Government in filing denaturalization proceedings in this type of case. It reverses a long line of cases in the lower federal courts 1 and disregards a consistent administrative practice of over thirty years standing, a period which *101includes two recodifications of the immigration laws. Furthermore, the identical point on which the case today is decided was present in two earlier cases where it apparently was not considered important enough to be presented to this Court.2

The only authority for the Court’s action is an unpublished, informal, and somewhat ambiguous inter-departmental letter of the Attorney General written in 1907. While any Attorney General might well be proud to see his views given such lasting effect, he undoubtedly would be surprised to learn that the authority of such an informal statement could overrule later court decisions and a thirty-year, firmly established position of the Department of Justice. Many cases witness the fact that the Court has often given little or no weight to carefully drawn opinions of the Attorney General on questions of statutory interpretation.

But my major objection to the decision today is the extreme burden placed on the Government in cases such as this. The Court construes § 340 (a) of the Immigration and Nationality Act to require something more than was filed in the present case. The complaint here was verified; the Assistant United States Attorney swore that the facts alleged in the complaint were “true” and “that the sources of his information . . . are the records which are on file in his office.” Thus the complaint in this case was supported by the jurat of a notary public *102and the sworn statement of an Assistant United States Attorney who held office by virtue of his oath to carry out his official duties fairly and impartially. No more was present in Schneiderman v. United States, 320 U. S. 118.

The complaint alleged that Zueca had lied in his alien registration statement in 1940, and in his preliminary naturalization examination and testimony before a naturalization examiner in 1952. He is alleged to have stated under oath that “he did not belong to and was not associated with any organization which teaches or advocates the overthrow of existing government.” The complaint then alleges that Zueca “was a member of the Communist Party of the United States, including the Workers Communist Party . . . from 1925 to 1947.” Under § 305 of the Nationality Act of 1940, the complaint continues, Zucca’s naturalization was illegal.

But the majority declares that these sworn allegations are insufficient. It makes a vague reference to the pleading of “ultimate facts while the affidavit must set forth evidentiary matters showing good cause for cancellation of citizenship.” From this statement I can draw only one conclusion. As respondent contends, “good cause” means “that the Government must furnish the Court with sworn statements by persons having personal knowledge of the facts . . . Congress could not have intended that the courts be required to accept the second-hand statements of investigators . . . .” 3 Apparently neither the United States Attorney nor anyone in the Immigration Service or the FBI can make such an affidavit unless he has personal knowledge of the facts. This would limit verification by these officers to cases based on prior undis*103closed criminal convictions, arrival age where controlling, etc.

Bnt in proceedings based solely on membership in the Communist Party substantially different conditions prevail. Invariably, membership can be proved only from the testimony of other members concerning attendance at meetings, payment of dues, etc. There are no membership cards in the Party and have been none for more than a decade. If these evidential methods of proof — the testimony and identity of undercover agents — must be disclosed in an affidavit, the Government must choose between foregoing denaturalization cases and drying up its source of information before the proceeding can be brought. It is common knowledge among law enforcement officers that witnesses are affidavit-shy, particularly in cases involving subversion. Often, testimony can be obtained only in court with the aid of compulsory process. The difficulties in requiring exposure by affidavit are overwhelming and decisive in cases of this type.

I do not believe Congress ever intended such a rule. To me § 340 (a) is clear and unambiguous. Its plain reading is that proceedings may be filed by the United States Attorney “upon affidavit showing good cause therefor.” Here the Attorney swore to specific charges which certainly do constitute “good cause.” The sworn statement that petitioner was a member of the Communist Party and the Workers Communist Party from 1925 to 1947 alleges a prima facie case. To me it seems obvious that the purpose of § 340 (a) — to reduce the possibility of spurious denaturalization proceedings — is fully served by such a sworn statement.

Nor did the Congress intend that there should be two trials of issues of fact in these cases. To require the filing of evidential affidavits implies, as Zueca contends, extensive testing of their sufficiency before trial. The defendant is thus given two chances at the Government’s case. *104There is no comparable requirement for an indictment in a criminal prosecution, Costello v. United States, 350 U. S. 359. But denaturalization is only a civil proceeding to withdraw a privilege wrongfully obtained. There has always been the requirement of proof under cross-examination of charges against the naturalized citizen, but apparently in this case the Court authorizes an additional procedure. Before his trial in this denaturalization proceeding, Zueca may file a bill of particulars and take depositions of each witness signing a “good cause” affidavit. The scope and reliability of the affidavits are then made the subject of judicial inquiry.4 If the Government proves its case at this stage, it may then go to trial, where the same evidence is considered again. To my way of thinking, this clearly frustrates an important congressional program, a part of the broader one designed to protect our country from Communist infiltration.

In my opinion § 340 (a) requires the United States Attorney to allege in a sworn complaint sufficient factual information to show a prima facie case for denaturalization. At most it should be sufficient for an officer of the Immigration and Naturalization Service familiar with the case to make such allegations under oath. If the Court would require this paper to be headed “Affidavit” and contained in a separate blue backing, I would have no objection, though I see nothing to be gained from such a technical labeling. But as I read the Court’s decision today it goes much further than this, and it may well submerge the denaturalization procedure established by Congress in a morass of unintended procedural difficulties.