This article examines a science teacher's use of and reflections on classroom talk in teaching a unit on genetics on a bilingual education programme. Constructivist, sociocultural and discursive psychological perspectives on conceptual change and classroom talk are reviewed. Data are drawn from three sources: preactive interview, video-recording of classroom interaction, and video-based postactive reflections. Detailed analyses of transcripts show that even when the teacher oriented to the constructivist strategy of eliciting students' views, there were missed opportunities to use a more dialogic approach. Implications for teacher education of science teachers in first and second language contexts are discussed. (Contains 2 tables.)