This study explores teachers' use of Spanish in sixth-grade bilingual science classrooms at an urban middle school in Texas. We used a case study approach to study how two Spanish-English bilingual teachers utilized Spanish during instruction in a unit on space exploration and their beliefs on Spanish use in English-dominant science classrooms. The results showed that the teachers used Spanish mainly for classroom management purposes like redirecting behavior and reiterating instructions previously given in English. There were inconsistencies between teachers' practices and beliefs regarding Spanish use. The school's English-centered policy and a lack of validation of language as a resource for science learning hindered the use of Spanish as a tool for science learning. The implications of the findings for teacher professional development were discussed.