In this paper, we draw on Wenger's (1998) conception of communities of practice to observe the emergence of a community of practice among middle grades mathematics teachers who participated in a two-year blended-format (online synchronous, online asynchronous, and face to face) professional development program designed to increase middle-grades mathematics teachers' content and pedagogical content knowledge, as well as foster the evolution of local communities of practices. Results indicate that the professional development program supported the development of a community of practice among a subset of participating teachers, while others failed to develop what constitutes a community of practice. Issues related to professional development design in light of these findings are discussed.