Introduction: Autism spectrum disorder (ASD) is a prevalent neurological disorder in children characterized by restrictive, repetitive patterns of behavior that place an added burden on everyday functions. Aerobic exercise has the propensity to reduce stereotypic behaviors in children with ASD. This study sought to quantify the acute effect of exercise and to assess the influence of duration and intensity on the frequency of stereotypic behaviors in children with ASD. Methods: Participants in this study (N = 7, M-age = 13.0 +/- 1.4 yr, M-height = 1.64 +/- 0.01 m, and M-weight = 60.1 +/- 13.7 kg) underwent five separate days of treatments, including a control condition (C), a low-intensity 10-min condition (10L), a high-intensity 10-min condition (10H), a low-intensity 20-min condition (20L), and a high-intensity 20-min condition (20H) in which intensity was quantified using HR as well as RPE. Before and 60 min after exercise, the frequency of stereotypic behaviors was recorded. Results: Results indicated a reduction in behaviors in response to exercise compared with the C trial throughout all conditions except 20H. Interestingly, the most exhaustive exercise session led to increased stereotypic behaviors at all postexercise periods compared with the other exercise trials (P < 0.10). The 10L condition showed the greatest reduction at 60 min postexercise compared with the 20H or the control trial" s response (P < 0.05). Examining the behavioral responses to exercise using effect sizes indicated the 10L condition showed the greatest reduction in frequency throughout all four time points (ESrange = -0.87 to -1.03) compared with baseline. Conclusion: Although it appears high-intensity aerobic exercise may exacerbate stereotypic behaviors in children with ASD, low-to moderate-intensity exercise produces significant and large reductions in these behaviors. This provides an easily administered and cost-effective way to positively impact these individuals.