Optimum development of both bone mass and bone quality during childhood is crucial in prevention of osteoporosis, a condition in which fractures occur late in life. Increasing physical activity is one modality that shows promise for augmenting bone mass and improving bone architecture in adults. Although a positive association has been reported between physical activity and bone densitometry measurements in children, no studies have reported the relationship between physical activity and ultrasonic transmission in bone in children. The purpose of this study was to determine, in a cross-sectional sample of children, whether self-chosen levels of physical activity are associated with variation in apparent velocity of ultrasound (AVU). The convenience sample included 65 third- and fourth-grade girls and boys from a rural Midwestern county of the United States. AVU was measured with the Signet (Osteo-Technology, Cambridge, MA). Physical activity was determined with the TriTrac-R3D Ergometer (Hemokinetics, Madison, WI). A significant negative correlation (r = -0.54; p = 0.0007) was found between the girls' activity and their AVU values. Stepwise regression analysis indicated activity to be the strongest predictor of AVU in girls, accounting for 28% of the variance (p = 0.0005). Weight also was a significant predictor, accounting for an additional 13% of the variance (p = 0.00001). We conclude that more active prepubertal girls have lower AVU values than less active girls. This negative correlation between activity and AVU is curious, especially since other studies have found a positive relationship between activity and bone density in children. We speculate whether a lag in development of bone quality might result in excessive microdamage in more active children. Further studies are needed to test this hypothesis.