Promoting youth physical activity is an important health goal for the nation. To promote physical activity, interventionists and practitioners need to have a clear understanding of physical activity patterns. To date, few longitudinal studies have assessed objectively measured physical activity and no longitudinal studies have assessed pedometer-determined physical activity. The primary purpose of this study was to determine how pedometer-determined physical activity changes from mid elementary (grades 3 & 4) to late elementary (5 & 6) and late elementary (5 & 6) to junior high (7 & 8). Complete data was collected on 206 students at baseline (grades 3-6) and 27-month follow-up (grades 5-8). Data were collected up to four weekdays for elementary and up to eight weekdays for junior high. Yamax Digiwalker SW-200 (Tokyo, Japan) and Walk4Life My Life Stepper 2525 pedometers (Taiwan) were used to assess physical activity at baseline and follow-up, respectively. A three-way mixed design analysis of variance (ANOVA) was conducted to determine if children’s level of physical activity changes over 27 months and if these changes are different based on sex and grade. A 2 (Time; baseline, follow-up) x 2 (Sex) x 4 (Grade; 3, 4, 5, 6) design included time as the within subjects factor, sex and grade as the between subjects factors, and mean daily step counts (DSC) as the dependent variable. Significant differences were found on the time by grade interaction, Wilks's Ë = .90, F(3, 198) = 7.06, p < .001, partial ç² = .10. Four dependent sample t tests (by grade) between baseline and follow-up DSC were conducted as follow-up analysis. Students transitioning from late elementary to junior high experienced a slight decline in DSC (126-197), although not significant. Students transitioning to late elementary experienced a significant (p < .001) increase in DSC (1830-1963). The time by grade interaction indicates a clear divergence between physical activity levels between students remaining in elementary and those transitioning to junior high. Understanding youth physical activity patterns is important because interventionists can potentially target specific grades in which children and adolescents show distinct declines in physical activity. Based on the current evidence, interventionists and practitioners may need to intervene prior to and during junior high school in an effort to maintain and increase physical activity.Keyword(s): middle school issues, physical activity, research