The current study examines the effects of prekindergarten quality and quantity on externalizing behavior problems for children living in non-parental care, compared to other children from socioeconomically at-risk backgrounds. Data were obtained from the Head Start Impact Study. Non-parental care was defined as a primary caregiver other than a biological, adoptive, or step-parent. The sample included 3029 children who attended center-based prekindergarten. Teacher-child conflict and more hours of prekindergarten predicted increased externalizing behavior problems for the full sample. Teacher-child closeness and overall process quality were only associated with externalizing behavior for children in non-parental care. Findings are discussed within a goodness-of-fit perspective in which the vulnerabilities of children in non-parental care explain how they respond to their prekindergarten experiences. (author abstract)