This research describes the biliteracy learning of young bilingual children in an English as a Second Language classroom. In particular, it explores factors influencing their biliteracy in a context that provided systematic and formalized instruction only in English. Using a holistic perspective on bilingualism and sociocultural theories of learning, this study analyzes children's writing and writing-related talk. In particular, this study draws on Vygotsky's notion of "scientific" and "spontaneous" learning. The findings suggest that children used concepts that they had been systematically taught and applied them to new circumstances, that they drew on support from their families outside school, and that their interactions with each other supported their biliterate development.