This thesis reports on a study of the use of first and second languages among students of a Spanish immersion program. Eight fifth- and sixth-graders with varying levels of Spanish proficiency participated in the study. Data were collected through student verbal reports, questionnaire-based interviews, classroom observations, and background information such as school grades and achievement test scores. The study found that although students used Spanish when talking to the teacher in teacher-fronted classrooms situations, they were unlikely to use Spanish when talking to fellow students, unless the students were working on an academic task. Learners were able to understand written and verbal instructions in Spanish, especially if these instructions were over material easily grasped. However, learners had more difficulty understanding instructions in Spanish if the material was new or difficult for any reason. (Contains 41 references.) (MDM)