BACKGROUND:Parents are influential in supporting children's physical activity, but relatively little is known about the role of fathers in children's physical activity. METHODS:Semi-structured interviews (n = 24) were conducted with low-active and active fathers of children 3-11 years old. Deductive thematic analysis was used to identify fathers' physical activity practices and understand how fathers interact with their children around physical activity. RESULTS:All fathers demonstrated coparticipation in physical activity with their children. Other physical activity practices commonly used by fathers included as follows: facilitation of active opportunities, modeling, involvement through coaching or teaching, and encouragement. In addition, fathers viewed physical activity as an opportunity to spend time with their children to bond and develop shared interests. Finally, fathers perceived their role in children's physical activity to be different compared with mothers. Regarding father activity level, active fathers discussed modeling more frequently and tended to engage in a variety of different activities compared with low-active fathers. CONCLUSIONS:Fathers play an important role in their children's physical activity, suggesting that physical activity may be one context in which to prompt paternal involvement, foster father-child relationships, and strengthen paternal parenting.