We examined predictors of leisure-time physical activity intention and behaviour in children, and the potential direct and moderating effects of gender and ethnicity. Participants were 364 (230 Asian; 134 Caucasian) 9-11-year-old children who completed measures of the theory of planned behaviour (TPB) and physical activity behaviour (Physical Activity Questionnaire for Children) across three, 3-month intervals (i.e., two prediction time-periods) in Canada. The TPB explained 35-50% of the variance in physical activity behaviour and 74-76% of the variance in intention using structural equation modelling. An autoregressive longitudinal path model showed that the TPB predicted changes in physical activity and physical activity predicted changes in TPB constructs (p<.05). Overall, intention and perceived behavioural control were significant (p<.05) contributors to the direct prediction of behaviour and subjective norm and perceived behavioural control were significant (p<.05) predictors of intention. Gender did not show a robust relationship with physical activity or intention, but the effect of ethnicity on physical activity was mediated through perceived behavioural control (Asians less active than Caucasians). Finally, gender and ethnicity did not moderate TPB and physical activity relations (p<.05). These data suggest that childhood may be a formative stage for the development of physical activity cognitions and that promoting physical activity in children may require normative and control-based intervention.