The purpose of this study was to examine the pedometer-measured physical activity levels of high school students (Grades 9–12). Comparisons were made between sexes, among grades, among groups based on level of participation in sport and physical education, and among groups based on levels of self-reported physical activity (based on questions from the National Youth Risk Behavior Surveillance System). Participants wore sealed pedometers for 4 consecutive school days. Results indicated no differences among days of monitoring but did show significant differences in mean steps per day between sexes, among grades, and among activity levels. Males took more steps per day than females did, and 10th graders took more steps than 12th graders did. Teens involved in sport and physical education took more steps than did those not involved. Teens who reported meeting both moderate and vigorous activity recommendations were most active, followed by teens meeting recommendations for moderate activity.