For at least two decades, studies have demonstrated that the least experienced and credentialed teachers are concentrated in poor, minority, and low-performing schools. Some blame provisions in collective bargaining agreements (CBAs) between teachers unions and school districts that favor senior teachers. Seniority preference rules, they say, exacerbate the "teacher quality gap" by allowing experienced teachers to transfer. Using data from Florida, the authors analyze whether and how CBAs influence the distribution of teacher quality within school districts, paying special attention to staffing rules that grant preferences to senior teachers. They find little evidence that the within-district variation in teacher quality between more and less disadvantaged schools in Florida is explained by the determinativeness of union contract rules. (Contains 10 notes, 5 tables, and 1 figure.)