In the present research, the influence of racial diversity among classmates and friends on changes in racial self-identification among multiracial youth was examined (n = 5,209; M[subscript age] = 10.56 years at the beginning of sixth grade). A novel individual-level measure of diversity among classmates based on participants' course schedules was utilized. The findings revealed that although there was some fluidity in multiracial identification at the beginning of middle school, changes in multiracial identification were more evident later in middle school. In addition, although diversity among classmates and friends both increased the likelihood of multiracial identification in the beginning of middle school, only diversity among friends mattered later in middle school, when fluidity in multiracial identification was at its peak.