A withdrawal design was used to investigate how physical activity affects on-task behavior of young children with significant developmental delays in a special education preschool classroom. Five preschool age children with significant developmental delays engaged in either physical activity or seated center activities for 20 min prior to a 15-min teacher-directed group activity. Momentary time sampling was used to calculate the percentage of intervals the participants were on-task using 15-s intervals. Results indicated all of the participants’ on-task behavior was higher during the physical activity condition. These findings suggest physical activity may be used as a proactive behavioral intervention to improve the on-task behavior of young children with significant developmental delays during teacher-directed group activities.