Current research on cross-language transfer has focused primarily on literacy development. The present study extends the scope of existing research by focusing on argumentative reasoning, an important but under-research academic skill. A single-case study with a reversal design was conducted with four Chinese-English elementary students with the well-established Collaborative Reasoning (CR) discussion model. Data included transcripts of the lessons and discussions, surveys, and interviews. Cross- language transfer of argumentative reasoning was observed at the lexical, syntactic, and discourse levels and varied with factors identified in previous research on biliteracy development. Theoretical and methodological contributions of the study are discussed.