Teachers' social networks can play an important role in teacher learning and organizational change. But what influences teachers' networks? Why do some teachers have networks that are likely to support individual and organizational change, while others do not? This study is a first step in answering this question. We focus on how district policy influences the quality and configuration of teachers' social networks. We draw on a longitudinal, qualitative study of implementation of a mathematics curriculum in four schools. We show that district policy (1) shaped the tie formation process, influencing the structure of networks; (2) mobilized resources that teachers subsequently accessed via their networks, influencing the benefits accrued through network exchanges; and (3) introduced interaction routines that interrupted conventional ways that teachers talked together. We thus uncover heretofore unexplored facets of network formation and change. We also provide insight into dimensions of social networks that are amenable to outside intervention.