This paper presents the first in a series of studies on the informal advice networks of a community of teachers in an in-service professional development program. The aim of the research was to use Social Network Analysis as a methodological tool to reveal the social networks developed by the teachers, and to examine whether these networks maximized teachers' access to practitioner-based social capital. We define practitioner-based social capital as the knowledge and resources for teaching practice that are accessible through a social network. Our findings indicated that teachers did not naturally build advice networks that would cultivate the highest levels of practitioner-based social capital. This study offers new possibilities for social network research on teachers' learning communities in understanding the dynamics of teacher networks. Further, program facilitators will use this information to strategically foster improved teacher collaboration and learning, which will be described in forthcoming studies in this series. (Contains 4 tables and 2 figures.)