Understandings of the ways home and school languages shape classroom dynamics and influence development, identity, and subsequent school success are important for teachers of both bilingual and African American students. This article builds a link between these complementary bodies of research by analyzing interactions in a second grade mainstream classroom in which the language development of bilingual and African American children were simultaneously relevant. We focus on two qualitatively different kinds of classroom language use: when instruction was solely in English, and when Spanish became a tool for instruction. Our findings suggest that the latter language practice subsequently marginalized the participation of English monolingual students; this especially affected the African American students in the classroom, who were interactionally delegitimized as participants in bilingual interaction despite their desire to participate in both languages. This study suggests the need to ensure that multilingualism is brought into the classroom as a resource for all students. Recognizing this need, however, necessitates interdisciplinary research that crosses the fields of second language acquisition, bilingual education, and sociolinguistics. Such disciplinary boundary crossing can usefully inform teachers and researchers looking for new understandings of language learning in contemporary classrooms.