In this article we examine a teacher-authored narrative in which Steve Mann (2002), an English as a second language teacher, critically reflects on how his teaching beliefs were transformed as a result of participating in "cooperative development" (Edge, 1992, 2002). Cooperative development is an inquiry-based approach to professional development that promotes self-development as it occurs within the context of a supportive group of colleagues. Our analysis of Mann's narrative--based on a Vygotskian sociocultural view of cognitive development--exposes evidence that unexpected aspects of the dialogic process that Mann and his colleagues engaged in spurred his cognitive development. Mann's conceptual development is traced from the initial gap between his cognition and emotions, manifested in a contradiction between his beliefs and practices, through the dialogic processes of externalization and reconceptualization, and towards a transformation--he asserts--in his teaching practices. By externalizing these contradictions with his colleagues, who act as temporary others, he is in essence operating in a zone of proximal development in which the mediational means provided by the group enable him to reconceptualize and restructure his beliefs and work towards self-regulation of his teaching practices.