This phenomenological study (Creswell, 2003, 2007; van Manen, 1997) explores student and teacher perceptions of first language use in French immersion mathematics classrooms at a large, urban high school in Canada. During individual interviews, participants discussed their perceptions and experiences of French immersion mathematics, language use, and, in particular, first language use. Interview data are analysed through a sociocultural theory lens (e.g., Lantolf, 2000; Swain & Lapkin, 2000), drawing on key notions such as language interdependence and the use of language as a cognitive tool. While first language use in second language classrooms remains controversial, this article contributes to an open discussion on the potential role students' first language can play in determining effective language and content learning.