This research describes language use by four first-grade students during mathematics and Language Arts instruction in a one-way 50/50 Mandarin immersion classroom. The urban public school was situated in the heart of an African-American community in the Pacific Northwest region of the United States. Participants were video- and audio-recorded while wearing lapel microphones for 56-75 minutes per week for five weeks, followed by a semi-structured focus group interview. A total of 3,090 speech turns were coded and analysed under five categories: number of speech turns, vocabulary, grammar, linguistic functions, and other themes that emerged from the interview. Overall, students used Mandarin 61% of the time. Data indicated that multiple factors may impact student target language use, including motivation, learning strategies, social identity, linguistic background and pedagogy. Implications for changes in immersion curriculum and instruction, as well as calls for future research on trilingual education are shared.