Encouraging student prosocial behavior (PSB) is a challenge for urban middle schools. The issue of student behavior is a racialized one, as Black students generally evince more negative behavioral outcomes than their White peers. This racial "behavior gap" may be conditional on the school environment. This study examines how one element of the school environment--racial composition--affects PSB, drawing on a sample that includes approximately 2,000 Black students and 1,400 White students in 11 urban middle schools in the Southeastern United States. Results of multilevel regression models show that the effect of racial composition on PSB is different for students of different races. As the proportion of Black students in a grade cohort increases, the gap in PSB between Black and White students shrinks and becomes insignificant. The closing of the gap is driven mostly by the declining PSB of White students, while Black students' PSB stays constant. Implications for school practice are discussed.