There have been limited prospective studies investigating physical activity and physical self-perceptions in children. In this investigation, mean steps/day did not significantly change from late elementary to junior high for either boys or girls; however, boys accumulated more steps both at baseline and follow-up. Physical self-perception measures were significantly related to changes in steps/day and accounted for 21% (15% adjusted) of the variance after controlling for demographic/biological variables, R[superscript 2] change = 0.21, adjusted R[superscript 2] change = 0.15, F(5, 88) = 4.66, p less than 0.001. Sport competence emerged as the most important predictor, t(88) = -3.76, p less than 0.001, and was inversely related to physical activity change. The lack of opportunities to participate in sport activities normally found at the onset of adolescence may help explain this unexpected result. (Contains 5 tables and 1 figure.)