As content-based instruction (CBI) programmes involve certain degrees of integration of second language (L2) and content learning, collaboration between L2 and content subject teachers has been strongly advocated. Whether this type of teacher collaboration can be successfully implemented will depend on many factors, one of which is teacher beliefs and attitudes. This study compares the beliefs and attitudes of L2 teachers with those of content subject teachers regarding their roles in CBI and cross-curricular collaboration. Data collected from the 261completed questionnaires, complemented with six semi-structured interviews, administered to teachers in Hong Kong secondary schools which practise CBI, show that the two groups of teachers have significantly different beliefs about their roles concerning language teaching in CBI. Even though they generally believe in the effectiveness of collaboration, they perceive some potential problems and are not fully committed to it. Such findings have important implications for the feasibility of cross-curricular collaboration, which is a practice often advocated to facilitate L2 learning in CBI. The findings can also shed light on teacher education in CBI.