Teacher-student discourse continues to be teacher-centred even though researchers and reform documents have recommended changes toward increased levels of student-centred discourse. In science education this situation is paralleled by effort to make scientific inquiry more student-centred. The purpose of this study was to investigate how discourse forms changed over time in a classroom where the regular teacher and his students were scaffolded in the transitioning to student-centred scientific inquiry. Video-recordings were collected at intervals over one academic year. Three prominent forms of discourse were identified: two teacher-authoritative forms and one more interactive, dialogic form. As the lessons increasingly turned into student-centred scientific inquiry, a shift to the dialogic discourse form was found. Co-teaching provided for (a) guidance towards an organisation of events in the classroom that included regular teacher-student dialogue (b) modelling of the more dialogic form of discourse.