The Federal Credit Union Act (FCUA) authorizes the chartering of credit unions at the national level. Under § 109 of the FCUA, federal credit union membership is limited to groups having a common bond of occupation or association, or to groups within a well-defined neighborhood, community, or rural district. Pursuant to its interpretation of this provision, the National Credit Union Administration (NCUA) (defendant) has permitted federal credit unions to be composed entirely of unrelated employer groups, as long as these groups have their own common bond of occupation. AT&T Family Federal Credit Union (ATTF) expanded its operations under this interpretation and added several unrelated employer groups as members. Five commercial banks (plaintiffs) challenged the NCUA’s approval of ATTF’s membership expansion under the Administrative Procedure Act (APA). The United States Supreme Court granted certiorari.