Language is the means through which mathematics is learned and mathematical reasoning is developed and expressed. Students' development of mathematical knowledge is dependent upon their codevelopment of language competencies. This study sought to understand the intersection of language acquisition and mathematical reasoning in a multigrade, dual-language mathematics classroom. The focal research question was: In what ways do immigrant middle-school students use L1 and L2 to communicate and to reason mathematically? The students who participated were first- and second-generation immigrants from Mexico. Data included (1) ethnographic field notes, (2), videotape and audiotape of small-group work, (3) interviews with the bilingual mathematics teacher, and (4) student work. Findings suggest that (1) nonroutine mathematical problem posing is useful for eliciting productive talk and encouraging reasoning; and (2) when given the opportunity, ELLs are likely to draw productively upon their linguistic resources in L1 and L2 to support high-level mathematical reasoning. (Contains 1 figure and 1 endnote.)