This paper explores teachers' "agency" as they talk about using a Sociocultural approach to teaching and learning (Collective Argumentation) to mediate activity in the mathematics classroom. The paper examines a re-conceptualisation of teacher agency as evidence in a report by one middle school teacher of a classroom mathematics activity. Employing discourse analysis to examine aspects of teacher activity in the report, the paper relates the development of teacher agency to the appropriation of pedagogical practices and to teacher talk about those practices.