Background: After-school programs may provide valuable opportunities for children to accumulate healthful physical activity (PA). This study assessed the PA of third-, fourth-, and ?fth-grade children in the Keep It Moving! (KIM) after-school PA program, which was implemented in an ethnically diverse and low socioeconomic status school district in Colorado Springs, Colorado. Methods: The PA of KIM participating children (N = 116) at 4 elementary schools was objectively assessed using ActiGraph accelerometers and the System for Observing Fitness Instruction Time (SOFIT). Linear mixed-effects models or generalized linear mixed-effects models were used to compare time spent in sedentary (SED) behaviors, light PA (LPA), moderate PA (MPA), vigorous PA (VPA), and moderate-to-vigorous PA (MVPA) between genders and weight status classi?cations during KIM sessions. Results: Children accumulated 7.6 minutes of SED time, 26.9 minutes of LPA, and 22.2 minutes of MVPA during KIM sessions. Boys accumulated less SED time (p less than 0.05) and LPA (p = 0.04) than girls, but accumulated more MPA (p = 0.04), VPA (p = 0.03), and MVPA (p = 0.03). Overweight/obese children accumulated more LPA (p = 0.04) and less VPA (p less than 0.05) than nonoverweight children. SOFIT data indicated that children spent a considerable proportion of KIM sessions being very active (12.4%), walking (36.0%), or standing (40.3%). Conclusion: The KIM program provides opportunities for disadvantaged children to accumulate substantial amounts of MVPA (greater than 20 minutes per session) in an effort to meet current PA guidelines. (Contains 2 tables.)