The purpose of this study was to address three theoretical assumptions in an examination of the Spanish and English language proficiency and academic achievement of second-and third-grade English and Spanish speakers who were enrolled in a bilingual immersion program. Findings provided support for these three assumptions: (a) High levels of proficiency in the two languages will facilitate academic achievement; (b) there are two types of language proficiency, academic and communicative, and the extent to which one is proficient in these two types may vary; and (c) there is transfer of content across languages.