To address reduced physical education (PE) in elementary schools, a 12-week physical activity protocol was tested on 5–12-year-old, primarily African American, girls (n= 226) and boys (n = 344) at 14 YMCA after-school care sites. The 3 times/week, 45-min session curriculum included cardiovascular, resistance, and flexibility training, in which all children could participate simultaneously, and a behavioral skills education component. After-school counselors, formerly untrained in PE methods, administered the sessions, with periodic supervision by YMCA wellness staff members. Analyses of the eight Age x Sex subsamples indicated significant improvements on body composition, strength, and endurance, both within-groups and when predicted changes due to maturation were accounted for. Exercise barriers self-efficacy significantly increased in subsamples of 9–10- and 11–12-year-old girls only. The need for replication across ethnic groups was suggested. Limitations and the need for extension of research on supplementation of elementary school PE were discussed. © 2005 Taylor and Francis Group, LLC.