This article reports analysis of data collected from a quasi-experimental study in 2 Canadian late French immersion science classes. We examine if, how, and when the first language (L1) is used when students in the first years of their second language learning talk about complex science concepts. We compare differences in groups following a 2-month intervention. Specifically, we study differences in complexity of oral utterances, and differences in use of L1 in oral utterances after participating in an intervention using a literacy-based approach, or the typical, district-prescribed approach. Furthermore, we assess whether increased use of the L1 in complex content statements is positively associated with gains in French and science knowledge. Advanced statistical analyses linked the complexity of student utterances with language use, written skills in French, and performance in science. In this way, this article makes a significant contribution to the existing body of literature on this important topic.