Background: Relatively little is known regarding after-school behavior. This study examined after-school active and sedentary behaviors among youth participating in the Study of Early Child Care and Youth Development. Methods: An interview guided time-use approach was used to obtain detailed longitudinal information about after-school (3-6 PM) behavior of a mixed gender cohort (n = 886) at ages 9 and 11 yrs. Responses obtained in 15-min intervals were coded into 29 exclusive behaviors and separated into 3 main categories [moderate- and vigorous-intensity physical activity (MVPA), light-intensity physical activity, and sedentary]. Sedentary category was further divided into screen and nonscreen categories. A mixed ANOVA design was used to examine gender and age-related differences in MVPA, light-intensity physical activity, sedentary, screen, and nonscreen. Results: MVPA was higher among boys compared with girls (P < .001) and decreased from 9 to 11 yrs (P < .001). Overall, total sedentary time was comparable between boys and girls despite a difference in reported screen time (boys > girls; P < .001) and nonscreen time (boys < girls; P < .001). Total sedentary time increased from 9 to 11 yrs (P < .001). Conclusion: Engagement in after-school behavior appears to change during preadolescence. Additional research is needed to understand factors associated with the selection of active and sedentary behavior over time.