Purpose: The evidence is strong that the instability of teacher rosters in urban school settings has negative consequences for student learning, but our concern is with the opposite phenomenon--What is the value added to the organization when a school's teaching roster is stable over time? Our theory of teacher corps stability hinges on the claim that the stability of a teacher corps over time is a "sine qua non" that, under certain conditions, permits formation of the social capital needed to catalyze school effectiveness. Design/methodology/approach: We test this claim using longitudinal data from 72 schools in a large, urban southwestern US school district. We first identified a subset of 47 schools with either chronic teacher turnover (high, stable turnover) or a stable teacher roster (low, stable turnover) via school-level HLM growth modeling techniques. These classifications were then used as a covariate in a series of HLM growth models investigating its relationship to growth in structural, relational and cognitive social capital over time. Findings: Our findings sustain a claim of the importance of teacher corps stability. In our sample of urban schools, we found robust increases in the relational and cognitive dimensions of social capital over time in those schools with stable rosters. Furthermore, schools with chronic turnover were declining significantly in relational social capital, but no appreciable growth in structural social capital was found in either stable roster or chronic teacher turnover schools. Practical implications: Given the nature of teacher corps stability and its relationship to key organizational outcomes, school leaders play a central role in realizing teacher corps stability within their school. A certain amount of this effort must necessarily be focused on retaining a stable corps of quality, happy, committed teachers. However, building social capital concerns the active engagement of all actors; thus, school leaders need to think beyond retention to how the teachers that remain can play larger leadership roles in this process. Originality/value: Few studies have examined the positive benefits that can emerge in schools where the majority of teachers remain year after year. Collectively, the study findings suggest that teacher corps stability can provide fertile conditions for the development of social capital that has the potential to enhance school effectiveness and that its staff can leverage for school improvement.