The primary purpose of this study was to investigate the disparity in physical activity levels of elementary school-aged children with mental retardation (MR) in both school (physical education & recess) and out of school environments (after-school and weekend) compared to their peers without disabilities. The secondary purpose of this study was to determine if there is a relationship between physical activity and gross motor skills in children both with and without MR. Physical activity levels of 46 children, 9 with mild MR and 37 without, were recorded for 7 continuous days using an accelerometer, sampling every 15 seconds. Following the physical activity data collection, the balance and ball skill subsections of the Movement Assessment Battery for Children were assessed. Independent t-tests revealed that physical activity counts were significantly lower during recess for children with MR (c= 187.3) than children without MR (c=287.3) (P>.01). During inclusive physical education, children with MR (c= 188.1) still had significantly lower physical activity levels than children without MR (c=438.7) (P>.001). This disparity was also evident in the out of school environment where children with MR (c= 135.3) were engaged in less physical activity after school than children without MR (c=205.7) (P>.001) and where physical activity counts on weekends for children with MR (c= 135.3) were lower than children without MR (c=180.5) (P>.05). There were no significant correlations between physical activity and motor ability in children with MR and without. The findings of this study indicate that interventions to increase physical activity in individuals with MR need to begin as early as the elementary school years.