Chinese immersion programs have been increasingly popular in US schools. However, we have insufficient data on young English-speaking children's acquisition of Chinese as a second language in these programs, and specifically on social contextual variables systematically promoting or hindering Chinese language use. Taking a variationist sociolinguistic approach, this mixed-methods case study identifies interlocutor and task as central social variables that most significantly condition the use, and therefore the acquisition, of Chinese by second graders attending an early total one-way Chinese immersion program in the United States. The idiosyncratic social roles learners played with different interlocutors in carrying out classroom tasks and activities help account for their use of Chinese or English, with implications for future research on second language acquisition and pedagogy in one-way language immersion education.