The effects of social capital on school improvement make it important to understand how teachers forge, maintain, or forfeit collegial relationships. Two common explanations focused on formal organizational features and individual characteristics do not address how social capital accrues from informal dynamics of teachers' interactions in communities. Our longitudinal study of teacher networks in four urban public schools finds that teachers in larger communities and communities with stronger cohesion are more likely to interact with each other over time. Teachers who frequently span community boundaries are less likely to continue interacting. These community-level characteristics are stronger predictors than teacher traits and formal organization. Our results have implications for how schools can support teachers in maintaining relationships and generating social capital.