The purpose of this study was to examine bidirectional relationships between student-teacher conflict and externalizing behavior in elementary school. A national sample of 9,663 kindergarten – third grade students (49% female; 51% White, non-Hispanic, 10% African American, 25% Hispanic, 9% Asian students) from the Early Childhood Longitudinal Study – Kindergarten Cohort of 2010-2011 were included. Using random intercept cross-lagged panel models, results indicated different patterns across sex. For boys, conflict demonstrated positive relationship with later externalizing behavior; whereas, for girls, there was not evidence of any cross-lagged effects. The article concludes with a discussion of the theoretical and practical implications of these findings. (author abstract)