While there has been increased interest in studying the notion of peer contagion as it relates to aggressive behaviors in elementary and middle schools, relatively little research has focused on the dynamics underlying peer contagion of aggressive behaviors during school recess. The current study analyzed whether aggressive behavior could spread between successive recess periods through peer contagion, with aggressive acts in early recess periods causing heightened aggression in later recess periods. Researchers observed the aggressive behaviors of boys during three successive recess periods at an urban elementary school using a systemic observation checklist that included hitting, kicking, pushing, tackling, throwing objects, and using offensive language. Results of structural equation modeling indicate support for the hypothesis that aggressive behaviors can spread through successive recess periods through peer contagion. Specifically, the second recess period mediated the relationship between aggressive behaviors in the first and third recess periods. Implications for practice are discussed with a focus on the importance of programs designed to curtail aggressive behaviors before they become prey to peer contagion. Table, figures, notes, references