This article explores student mobility in the Milwaukee Public Schools (MPS) and its effects on student achievement. An urban district with plentiful opportunities for school choice, Milwaukee has a transient student population. From 2003-04 through 2007-08, 11% of MPS students switched schools or left the district between the fall and spring of a given school year, while 22% were mobile between the spring of one year and the fall of the following year. Using both student fixed-effects and instrumental variables approaches, we examined the effects of this mobility on both the students who moved and their classmates who did not. We found evidence that mobile students’ test score gains dropped immediately after they switched schools, but these students typically recovered their losses in the subsequent year. We detected modest but statistically significant negative effects of grade-level turnover on non-mobile students’ academic achievement.