Objective: Examine the effect of the "California Children's Power Play! Campaign's School Idea & Resource Kits" for fourth/fifth grades on the psychosocial determinants of fruit and vegetable (FV) intake and physical activity (PA). Methods: Randomized, controlled trial (n = 31 low-resource public schools; 1,154 children). Ten grade-specific, 50-minute nutrition and PA lessons over an 8-week period. Pre/post surveys measuring knowledge, outcome expectations, and self-efficacy (SE) were analyzed using analysis of covariance, controlling for baseline values; chi-square comparing positive/negative changes, adjusted for cluster design effects. Results: Intervention children made significant gains for FV knowledge (4 items, P less than 0.05 to P less than 0.001); positive outcome expectations (fifth grade only, P less than 0.001); asking/shopping and eating SE (P = 0.04 and P less than 0.001); PA knowledge (2 items, P less than 0.01 to P less than 0.001); outcome expectations (P less than 0.05); and support seeking SE (P = 0.04); but not SE to overcome barriers. Conclusions and Implications: The "Power Play! Campaign's School Idea & Resource Kits" improved the determinants of FV intake and PA. (Contains 4 tables and 1 figure.)