In this paper, we draw upon a framework for analyzing the discursive interactions of science classrooms (Mortimer & Scott, 2003, "Meaning Making in Secondary Science Classrooms," Maidenhead, UK: Open University Press), to probe the movement between authoritative and dialogic discourse in a Brazilian high school science class. More specifically, we argue the point that such shifts between communicative approaches are an inevitable part of teaching whose purpose is to support meaningful learning of scientific knowledge. We suggest that a necessary tension therefore exists between authoritative and dialogic approaches as dialogic exchanges are followed by authoritative interventions (to develop the canonical scientific view), and the authoritative introduction of new ideas is followed by the opportunity for dialogic application and exploration of those ideas. In these ways, one communicative approach follows from the other, authoritativeness acting as a seed for dialogicity and vice versa. We discuss how this analysis, in terms of shifts in communicative approach, offers a new and complementary perspective on supporting "productive disciplinary engagement" (Engle & Conant, 2002, "Cognition and Instruction," 20, 399-484) in the classroom. Finally we consider some methodological issues arising from this study.