The government appeals from the district court’s judgment that the Immigration and Naturalization Service (INS) incorrectly denied Anau S. Fainga preferential status as the sister of Finau F. Mila, a naturalized United States citizen, in qualifying for a permanent visa. See 8 U.S.C. § 1153(aX5). Fainga, a native of the Kingdom of Tonga, was adopted by her mother’s sister (Mila’s mother) shortly after birth. Because Tongan law provides only for legal adoptions of children born illegitimately and Fainga was born legitimately, the adoption was according to Tongan custom. The INS interpreted the statute that defines “adopted children” as excluding Tongan children adopted by custom. The district court disagreed with the INS interpretation. The controlling issue on appeal is whether the district court gave too little deference to the INS interpretation.
To guide the INS in determining the order in which aliens subject to numerical quotas may become lawful permanent residents of the United States, Congress has set forth a system of priorities under which brothers and sisters of United States citizens are entitled to preferential treatment. See 8 U.S.C. § 1153(a)(5). To establish a brother-sister relationship, petitioner and the beneficiary of the preference must show they are “children” of a common parent as defined at 8 U.S.C. § 1101(b)(1). Matter of Clahar, 16 I & N Dec. 484, 485 (BIA 1978).
*125Adopted children must meet the requirements of 8 U.S.C. § 1101(b)(1)(E):
“(E) a child adopted while under the age of fourteen years if the child has thereafter been in the legal custody of, and has resided with, the adopting parent or parents for at least two years . ... ”
Petitioner has the burden of establishing that the purported adoption accords with the law of the country where it took place. Matter of Garcia-Rodriguez, 16 I & N Dec. 438, 439 (BIA 1978).
The INS district director does not dispute that Fainga was adopted by custom, but denied Mila’s petition because Fainga’s adoption was not a legal one. The Board of Immigration Appeals affirmed the district director’s decision and dismissed Mila’s appeal. On review the district court reversed and remanded, directing that Fainga be given preferential status because Tongan custom recognizes Fainga as the bona fide adopted daughter of Mila’s mother. The district court held the INS requirement of legal adoption was unduly restrictive and misinterpreted congressional intent, declaring that so long as a bona fide parent-child relationship exists, the INS has no reason to distinguish between a country that provides for adoption by law and one that provides for adoption by custom.
A federal court may reverse an INS denial of a preferential visa petition only if the INS abused its discretion. The INS abuses its discretion if it bases its decision upon an improper understanding of the law. Kaliski v. District Director of Immigration and Naturalization Service, 620 F.2d 214, 216 n.l (9th Cir. 1980). The Supreme Court has recognized that the construction of a statute by those charged with its administration is entitled to substantial deference. See, e.g, United States v. Rutherford, 442 U.S. 544, 553, 99 S.Ct. 2470, 2475, 61 L.Ed.2d 68 (1979). If reasonable and not contrary to the discernible intent of Congress, the agency’s interpretation should be approved even though it is not the only reasonable interpretation or the one the reviewing court would make if deciding the issue in the first instance. See Unemployment Compensation Comm’n v. Aragon, 329 U.S. 143, 153-54, 67 S.Ct. 245, 250, 91 L.Ed. 136 (1946); Nazareno v. Attorney General, 512 F.2d 936, 939-40 (D.C.Cir. 1975).
The INS interprets 8 U.S.C. § 1101(b)(1)(E) as requiring an adoption to accord with the laws of the country where it allegedly occurred. Matter of Garcia-Rodriguez, 16 I & N Dec. 438, 439 (BIA 1978). If that country has a legal procedure for adoption, the INS insists that the petitioner prove the adoption met those requirements. See Matter of Kong, 15 I & N Dec. 224 (BIA 1975); Matter of Kwik, 13 I & N Dec. 89 (BIA 1968). If the country has no formal adoption system, the INS recognizes customary adoption, but only if that country’s courts accept customary adoption as legally valid. The INS has recognized China’s customary adoptions because China’s legal system treats customarily adopted children as having rights equal in all respects to natural children. See Matter of Kwok, 14 I & N Dec. 127, 128-29 (BIA 1972) . But if the country does not legally acknowledge customary adoption, the INS, at least since 1961, has refused to find an adoption valid for immigration purposes. Matter of B, 9 I & N Dec. 521 (BIA 1961); accord, Matter of Palelei, 16 I & N Dec. 716 (BIA 1979); Matter of Benjamin, 15 I & N Dec. 709 (BIA 1976); Matter of Ashree, Ahmed and Ahmed, 14 I & N Dec. 305 (BIA 1973) .
The INS concluded that Tongan law does not recognize Tongan customary adoptions as legally valid and therefore denied Mila’s petition, relying upon Matter of Palelei, 16 I & N Dec. 716 (BIA 1979), also involving a Tongan customary adoption. That decision relied on a letter of the Tongan Crown Solicitor stating:
“There is no provision in our law for the adoption of children born legitimately. Nevertheless it has been a common practice in Tonga for relatives to raise and maintain children, including legitimate children as part of the family and to treat them in all respects as if they were legally adopted. Such ‘adoption’ does not give *126the child any legal right in the estate of the foster parent and is not recognized as legally valid under Tongan law.”
Letter from Crown Solicitor of Tonga to Ameriean-British Law Division, Library of Congress (Jan. 16, 1978) (quoted in Matter of Palelei, 16 I & N Dec. at 718). The INS emphasis has continued to be upon proof of the relationship, stressing need for a court decree or “legal recognition” of the adoption.
Appellees contend that the INS overemphasizes inheritance rights, which is particularly inappropriate in the instant case since no adopted child, whether legally or customarily adopted, may inherit under Tongan law. See Tonga Const. § 111; Law of Tonga c. 63, § 76(ii) (1967). Except for references by the dissent in Matter of Kong, 15 I & N Dec. 224, 226 (BIA 1975), we see in INS decisions no special emphasis upon inheritance rights of adopted children. See Matter of Ng, 14 I & N Dec. 135, 139 (BIA 1972) (customarily adopted daughter entitled to preferential status though Chinese law restricted inheritance to sons).
Although appellees can cite no authority holding Tongan customary adoptions legally valid, they maintain that Tongan customs are generally recognized in Tongan law. In support, they submit that in legal proceedings testimony of Tongan customs is admissible into evidence, Law of Tonga c. 13, §§ 5, 27 (1967), and that “assessors” sit with Tongan land courts to advise on customs relevant to disputes over land, Law of Tonga pt. IX, § 123. The amicus brief adds that in 1911, in the only reported Tongan case involving custody of a customarily adopted child, the adopting parents prevailed over a natural grandfather. Amicus Br. p. 18. But the single instance of a Tongan court awarding legal custody of a child to his customary adoptive parents rather than his natural grandfather does not negate the general rule stated by the Tongan Crown Solicitor. In spite of the general Tongan treatment of customs, Mila and Fainga do not claim that Tongan customary adoptions have the legal status under Tongan law required by the INS.
Furthermore, Mila and Fainga contend the INS interpretation of 8 U.S.C. § 1101(b)(1)(E) is unduly restrictive and thwarts the congressional intent of preserving the family unit by ignoring the genuineness of a family relationship. We agree that a broader interpretation would recognize and help to preserve some families that suffer under the INS interpretation. We think the instant case is a close one on the law. Since more people seek to immigrate into the United States than are permitted by congressionally established quotas, the INS interpretation benefits families with legally adopted children. It has the practical advantages of decreasing the likelihood of fraud, serving as a relatively bright line to determine which adoptions will be recognized for immigration purposes, and insuring that persons who bring adopted children into this country are legally responsible for their care. The legislative history is not so clear that we can say the INS interpretation is wrong. The INS has expressed its view in opinions issued at least since 1961, see Matter of B, 9 I & N Dec. 521 (BIA 1961), and since that date Congress has amended 8 U.S.C. § 1101 many times without changing the definition of adopted children. Thus, we hold that while the INS interpretation is not the only reasonable one, it is a reasonable construction and the district court erred in failing to defer to it.
Because we have found reasonable the INS interpretation of 8 U.S.C. § 1101(b)(1)(E), and Mila and Fainga do not contend that Tongan customary adoptions meet the standard set by that interpretation, the INS did not abuse its discretion in denying the petition.
494 F.Supp. 998. REVERSED.