Abstract: In the Grutter case, Justice O'Connor suggested that universities could justifiably try to enroll a "critical mass" of minority students. Enroll fewer than that "critical mass," reason some observers, and minority students will feel too marginalized to perform at their highest levels. In this article, we test whether minority students perform better with other students from their ethnic group in a class or school. To do so, we assemble data on the ethnicity and performance of each student in all classes at two law schools - for three years at one, and for 16 years at the other. Although these schools enrolled a smaller fraction of African-Americans than most law schools, they are located in states with a much smaller fraction of African-Americans than in the United States as a whole. There is also a large amount of variation in the percent African-American across classes. At these schools, we find no consistent evidence that having additional students from one's ethnic group raises a student's performance. Instead, we find some evidence that having additional ethnic peers lowers performance - albeit by a very small amount.