© 2015 Sports Medicine Australia. Objectives: To describe the change in afterschool levels of sedentary time and physical activity from childhood to adolescence. Design: Longitudinal. Methods: 375 youth (50% boys) from the Study of Early Child Care and Youth Development (United States) provided accelerometer data at 9 (2000/01) and 15 yrs (2006/07). Average time spent in sedentary, light, moderate, vigorous, and moderate-to-vigorous physical activity (MVPA; minh-1) was reported across the afterschool period (15:00-22:00) and separately across whole days. Mean comparisons were conducted to report between- and within-group differences in afterschool data. Analysis of covariance was used to determine the effect of sex, ethnicity, and baseline body mass index (BMI) on the 6-yr change in afterschool sedentary time and MVPA before and after adjusting for covariates (baseline afterschool sedentary/MVPA time, change in non-afterschool sedentary/MVPA time, difference in afterschool wear time, and socioeconomic status). Results: From 9 to 15 yrs, sedentary time increased and activity decreased during the afterschool period. After covariate adjustment, the decline in afterschool MVPA was significantly greater among girls, compared to boys (B coefficient (95%CI) = -0.94 (-1.47, -0.40)), and among overweight/obese youth, compared to youth with normal BMI values (B coefficient (95%CI) = -0.65 (-1.22, -0.08)). Conclusions: During the transition from childhood to adolescence, afterschool activity (minh-1) decreases while sedentary time increases. Programs are needed throughout this period that promote the maintenance of activity or encourage additional activity with age.