A substantial amount of school district policy is set in the collective bargaining agreements (CBAs) negotiated between teachers' unions and districts. Although previous studies have assumed that CBA provisions bargained by unions are a primary mechanism connecting union strength to outcomes for teachers and students, research has not yet addressed the basic question of whether stronger unions in fact bargain different CBAs. Combining data from a content analysis of agreements and a state-wide survey of school board members in California, this study examines whether stronger, more politically organized unions are associated with CBAs that place greater constraints on district policy options. The results show that contracts in districts with stronger unions, measured by both board members' evaluations of union power and union support of board members in recent elections, allow school district administrators less flexibility than do contracts in districts with weaker, less active unions. (Contains 18 notes and 5 tables.)