Recent federal, state, and district policies that have mainstreamed English language learner (ELL) students into general, English-only elementary school classrooms have raised questions among educational stakeholders about the widespread effects of these policies. Most research has focused on the outcomes of ELL students; almost nothing is known about the effects of these mainstreaming practices on non-ELL classmates, and no empirical research has evaluated their effects on socioemotional outcomes. The purpose of this study is to fill these research gaps by using quasi-experimental methods on a large-scale data set of kindergarten and first-grade students to examine the effects that mainstreamed ELL students have on 5 socioemotional scales for their classmates. The findings indicate a positive effect: kindergartners and first graders with a greater number of ELL classmates have lower problem behaviors and higher social skills. These findings are differentiated by individual characteristics and classroom contextual factors. Implications for policy and practice are addressed.