This paper reports a study of whole-class interactions in mathematics classrooms in a girls’ secondary school in Mauritius. It focuses on three teachers and their instructional language practices. Analysis of audio-recordings of lessons showed that code-switching was commonly practised by all the teachers in the study. The teachers’ comments on their use of language within the classroom show that although they are aware of the languages they use, they are not always conscious of their code-switching. Different functions of the teachers’ code-switching practices were identified, indicating it can be an important support for learning mathematics, despite some related tensions that teachers face in using code-switch in their teaching. The paper concludes with some implications for national policy and for teacher education.