While all mathematics is mediated by language, the role of language is especially complex in bi- and multilingual mathematics classrooms, and more so in bilingual education programs in which the explicit goals of both language and mathematics learning intersect. We explore bilingual French immersion students' linguistic and mathematical repertoires as they work through a series of probability problems. Focusing on the collaborative dialogue that occurred between students and researchers, our discourse analysis was informed by sociocultural theory and systemic functional linguistics. Findings indicate that students' linguistic and mathematical repertoires are intertwined, and that collaboration can offer opportunities for supporting bilingual learners' language and mathematics development. We conclude with implications and challenges for bilingual mathematics education. [For the complete proceedings, see ED583989.]