In dual immersion classrooms, students from different language backgrounds are immersed in the minority language for large portions of the school day with the expectation that they will become equally proficient in their first language (L1) and in their second language (L2). Research on dual immersion indicates that students reach above-average levels of academic achievement and linguistic proficiency, but, to date, very few quantifications have been made of how much of the minority language is used in these environments. Given that actually speaking a language is crucial for L2 acquisition as well as for heritage language maintenance, this study explored how much Spanish was used and for what purposes by 4 students (2 Spanish L1 and 2 two Spanish L2) in a Spanish English dual immersion classroom. Over 2,000 turns of natural classroom speech were recorded during a 5-month period. Overall, the students used Spanish 56% of the time, with 4 major trends: (a) The girls used more Spanish than the boys, regardless of L1; (b) the students averaged 82% Spanish when talking with the teacher, but only 32% when talking to peers; (c) Spanish was used primarily for on-task topics; (d) the students' English when speaking with peers covered a wider range of functions than did their Spanish. These findings lend support to proposals that a kind of diglossia exists in immersion classrooms (Tarone & Swain, 1995). Additional ethnographic data gathered through extensive participant-observations and interviews were interpreted using the concept of investment (Norton, 2000). Explanations of the students' language use are offered according to their sometimes competing identity investments.