This study used increased physical activity during recess to reduce stereotypy in preschoolers with Autism Spectrum Disorder. Results indicate increasing physical activity can be used as an intervention to reduce automatically maintained stereotypy in preschoolers with ASD. The intervention had a lesser effect on a preschooler whose stereotypy was maintained by attention. Results show topographies of the stereotypy were differentially affected for each individual. Not all motor stereotypies were affected equally within the same individual, and for one child, vocal stereotypies appeared to become less variable. Social validity measures indicate teachers would be willing to use this intervention in the future. Implications for using physical activity as an intervention for preschoolers with stereotypy are discussed. [The dissertation citations contained here are published with the permission of ProQuest LLC. Further reproduction is prohibited without permission. Copies of dissertations may be obtained by Telephone (800) 1-800-521-0600. Web page: http://www.proquest.com/en-US/products/dissertations/individuals.shtml.]