Technical Abstract: Objective: Assess the effects of simulated ostracism on children’s physical activity behavior, time allocated to sedentary behavior, and liking of physical activity. Methods: Nineteen (N=11 males, 8 females) children (11.7±1.3 years) completed two experimental sessions. During each session children played a virtual ball-toss computer game (Cyberball). In one session, children played Cyberball and experienced ostracism, in the other session they were exposed to the inclusion/control condition. The order of conditions was randomized. After playing Cyberball, children were taken to a gymnasium where they had free-choice access to physical and sedentary activities for 30-minutes. Children could participate in the activities, in any pattern they chose, for the entire period. Physical activity during the free-choice period was assessed via accelerometery and sedentary time via observation. Finally, children reported their liking for the activity session via a visual analog scale. Results: Children accumulated 22% fewer (p<0.01) accelerometer counts and 41% more (p<0.04) minutes of sedentary activity in the ostracized condition (8.9e+4 ± 4.5e+4 counts, 11.1 ± 9.3 minutes) relative to the included condition (10.8e+4 ± 4.7e+4 counts, 7.9 ± 7.9 minutes). Liking (8.8 ± 1.5 cm included, 8.1 ± 1.9 cm ostracized) of the activity sessions was not significantly different (p>0.10) between conditions. Conclusion: Simulated ostracism elicits decreased subsequent physical activity participation in children. Ostracism may contribute to children’s lack of physical activity.