This study examines the Spanish use of students and teachers at a US two-way immersion school. Students and teachers from Grades 1, 3, and 8 (5-6-year-olds, 7-8-years-olds, and 12-13-year-olds, respectively) were observed and interviewed, and students completed questionnaires to determine what factors influenced their language of choice and their divergence from Spanish when it was the language of instruction. Although students showed an overall preference for English, particularly in interactions with peers, findings indicate that students' language background, culturally relevant teaching activities, teacher language use, and students' sensitivity to others' need for language accommodation influenced their use of Spanish with peers. (Contains 4 notes.)