Bilingual and multilingual speakers mix two or more languages for communication, gaining information and sense-making in varieties of contexts. How do bilinguals mix languages and what does the mixing afford for their learning and participation? This study examines these questions, focusing on Korean-English bilingual youths in a community-based after-school science programme in the United States. A close analysis of videorecordings of the programme sessions demonstrates the ways in which bilinguals negotiate their participant positions and negotiate meanings in their study of science topics. The findings reveal that bilinguals utilise their Korean proficiency to craft diverse ways of participation and that linguistic and scientific sense-making co-occur and support each other. Based on these findings, I offer implications as to how science teachers should perceive students' mixing of multiple languages and how they can support bilinguals' learning of science content and English. Additionally, I suggest a future research direction that can help teachers to prepare for linguistically diverse science-learning and -teaching contexts.