The roles of first and second languages were studied, using ethnographic methods, in a middle school two-way bilingual education program that served Mexican-background students. The primary stated program goal was to develop bilingualism and biliteracy in both minority and majority language students. Classes were taught using English and Spanish on alternate days. The study focused on a core group of 21 Hispanic and 8 white students from working-class backgrounds. It was found that for academic purposes, the Hispanic students used English almost exclusively, and this is attributed to inattention to subtle aspects of program policy and a hidden curriculum. These included announcements made in English first, then Spanish, and the daily requirement that students learn an English vocabulary word but not a Spanish one. Student perceptions of the relative importance of English-medium and Spanish-medium tests were influenced by both the way in which students were prepared for them and the way the tests were administered. It was also found that instructional practices unwittingly devalued language-minority students' linguistic and cultural capital, through use of "high" rather than vernacular Spanish. It is concluded that despite good intentions, inattention to policy and practices can have unintended results, and that success through bilingualism and biliteracy is built on well-conceptualized and carefully monitored bilingual programs that give equal emphasis to minority languages and English. Contains 26 references. (MSE)