['Few studies have examined the mediators of behavior change in successful school-based physical activity interventions. The aim of this study was to explore potential mediators of physical activity in the Fit-4-Fun program for primary school children.', 'Group randomized controlled trial.', "Four primary schools were recruited in April, 2011 and randomized by school into intervention or control conditions. Participants included 213 children (mean age = 10.7 years ± 0.6; 52.2% female) with the treatment group (n = 118) completing the 8-week multi-component Fit-4-Fun program. Participants were assessed at baseline, 3- and 6-months. Physical activity was measured using Yamax SW700 pedometers (mean steps/day) and questionnaires were used to assess constructs from Social Cognitive Theory and Competence Motivation Theory. Hypothesized mediators measured included social support from peers, parents and teachers; physical activity self-efficacy (barrier and task); enjoyment; and perceived school physical environment. Mediation was assessed using Preacher and Hayes' multiple mediation regression SPSS macro. Action theory (A), conceptual theory (B) and the significance of the product of coefficients (AB) are reported.", 'The intervention had a significant effect on physical activity (p<0.001). The action theory test results revealed significant treatment effects at 3-months for perceived school environment (A=0.28, p<0.001); and at 6-month follow-up for perceived school environment (A=0.058, p<0.001), teacher social support (A=0.54, p<0.05) and enjoyment (A=-0.23, p<0.05). The conceptual theory test revealed a significant relationship between changes in teacher social support and changes in physical activity at 6-month follow-up (B=828, P<0.05). Teacher social support was shown to have a significant mediating effect on physical activity (AB = 445, CI = 77-1068 steps, proportion= 13%), and perceived school environment approached significance (AB = 434, CI= -415 to 1507 steps, proportion= 13%).'] Does social support from teachers mediate physical activity behavior change in children participating in the Fit-4-Fun intervention?