By developing two case studies of expert teaching in action, this study aimed to develop knowledge of talk in whole-class teaching in UK primary science lessons and understand this in relation to both the teachers' interpretations and sociocultural theoretical frameworks. Lessons were observed and video-recorded and the teachers engaged in video-stimulated-reflective dialogue to capture participants' reflections upon their own pedagogic purposes and interactions in the classroom. The analytic framework was developed at three levels: sequence of lessons, lesson, and episode. For each episode, the "communicative approach" and teaching purposes were recorded. Transcripts were developed for fine grain analysis of selected episodes and a quantitative analysis was undertaken of the use of communicative approaches. Findings exemplify how different communicative approaches were used by the case-study teachers for different pedagogical purposes at different points in the sequence of lessons, contributing to primary teachers' repertoire for planning and practice. The initial elicitation of children's ideas can be understood as pooling them to enhance multivoicedness and develop a shared resource for future dialogues. Whole-class talk can support univocality by rehearsing procedural knowledge and exploring the meanings of scientific terminology. Identifying salient features of phenomena in the context of the whole-class marks them as significant as shared knowledge but valuing other observations extends the multivoicedness of the discourse. (Contains 1 figure and 2 tables.)