Dual immersion classrooms combine students who speak a non-English language (in this case Spanish) with English speaking students learning the native language of the nonnative English speaking students. This case study recorded the output of Spanish first language (L1) and second language (L2) fifth graders over 5 months of Spanish language classes. The 2,203 turns of speech were coded according to nine sociolinguistic variables. Overall, students used Spanish 56 percent and English 44 percent of the time. Four major trends included the following: (1) girls used Spanish more often than boys, regardless of L1; (2) students averaged 82 percent Spanish when talking with the teacher but only 32 percent when talking to peers; (3) Spanish was mostly used for on-task topics (off-task social turns were made just 16 percent of the time in Spanish); and (4) students' peer English covered a wider range of functions (playing, teasing, and other off-task activity) than peer Spanish. Findings suggest that a kind of diglossia exists in immersion classrooms, with Spanish fulfilling mostly academic functions and rarely being used for socializing, and English being overwhelmingly preferred for social talk. Students who invested in identities as Spanish speakers more frequently spoke Spanish in the classroom, as long as no conflicts existed with their other identity investments. (Contains 37 references.) (SM)