There are few in-depth studies that have attempted to explore the complex relationshipsamong second language proficiency, foreign language aptitude, and intelligence. Information onsuch studies is usually found only in unpublished dissertations or in greatly reduced form injournal articles. This text is a rare example of a full-length report of such a study. Based on datafrom 160 EFL students in Japan, the researcher employed an extensive array of analyticalprocedures relating performance on English language proficiency measures with performance ona three-part aptitude battery and an intelligence measure. The proficiency measures includedmultiple-choice tests of grammar, cohesion, vocabulary, listening, and reading using short andlong texts as well as a cloze and a free composition. The foreign language aptitude battery was aJapanese version of a test modeled after the short version of the Modern Language Aptitude Test.The measures of verbal intelligence and reasoning came from an existing Kyoto University testwith 12 subtests.