This secondary analysis explored longitudinal associations between the concentration of shared-language peers and the development of prosocial and problem behavior in dual language learning (DLL) preschoolers from low socioeconomic backgrounds. Mixed-effects regression models were used to analyze year-end outcomes of 212 Spanish-speaking students in 73 Head Start classrooms in which English was the only instructional language. Research Findings: Classroom concentration of Spanish speakers was not associated with Spanish-speaking DLLs' year-end prosocial behavior. For problem behavior, there was a disordinal interaction between teacher social-emotional support and classroom concentration. In classrooms with higher teacher social-emotional support, the proportion of Spanish-speaking classmates was positively associated with Spanish-speaking children's problem behaviors. In classrooms with lower teacher social-emotional support, the proportion of Spanish-speaking students per classroom was negatively associated with Spanish-speaking children's problem behaviors. Practice or Policy: Findings highlight the value of shared-language peers in particular classroom contexts and have implications for DLL students' preschool classroom placements.