There is a dearth of research examining physical activity in children aged 6-10 years with low socioeconomic status, despite the fact there is good reason to suspect this is a critical period when physical activity habits are created. Physical activity and theory of planned behavior variables were measured at three time points, and children (N = 77) randomized to the experimental condition were additionally asked to form an implementation intention. Intention was a potent mediator of the past behavior-future behavior relationship and the implementation intention intervention significantly increased physical activity compared with the control condition. The findings suggest that physical activity can be increased in children aged 6-10 years with low socioeconomic status and that implementation intentions might enhance the effectiveness of children's physical activity programs. © 2010 Human Kinetics, Inc.