An unsustainable workload is considered the primary cause of teacher turnover at Charter Management Organizations (CMOs), yet most reports provide anecdotal evidence to support this claim. This study uses 2010-2011 survey data from one large CMO and finds that teachers’ perceptions of workload are significantly associated with decisions to leave across schools and teachers. About 1 out of 3 teachers who rated their workload "unmanageable" left their school compared with 1 in 10 who did not rate their workload unmanageable. However, controlling for perceptions of leadership and professional growth, workload was no longer associated with turnover. Accounting for measures of working conditions across schools and teachers, perceptions of the CMO’s student disciplinary systems were the only significant predictor of turnover.