This article presents data from a study examining secondary mathematics teachers' and science teachers' implementation of a language of instruction policy in Malaysia, which made English the medium for mathematics and science instruction. It explores the beliefs of math, science and language teachers, and how these beliefs influence their pedagogical practices in content-based language instruction classrooms. The study uses a mixed-methods approach for data collection and data analysis. Data is analysed using perspectives from content-based language teaching (CBLT) and from research on mathematics and science instruction for English language learners (ELLs). The results indicate that teachers' beliefs about their respective roles as only content teachers or only language teachers limit students' language learning opportunities. Factors such as curricular requirements, exam pressure and time constraints also shape classroom interactions, and have implications for student learning as well. The findings reveal the lack of collaboration between content and language teachers, and the need for sustained professional development concerning content and language integration for both groups of teachers. This study extends work on content-based language teaching to the previously unexamined Malaysian context. Its findings contribute to the ongoing work of improving instructional practices in content-based classrooms to integrate and maximize content and language learning for English language learners.