This study examined the effects of active supervision on the moderate to vigorous physical activity (MVPA) levels of middle school students during fitness instruction. Students from four separate classes, characterized as “low activity students,” and their teacher participated in the study. Students’ MVPA levels during fitness instruction served as the target behavior. Two supervisory conditions were contrasted using a reversal design. Passive supervision was characterized by low rates of teacher interaction, such as prompting, encouragement, feedback, and movement. Active supervision consisted of teaching patterns that included higher rates of interactions with students and movement. Mean MVPA levels during passive condition were 49.7% while mean MVPA levels during active supervision were 68%. Findings point to a functional relationship between specific components of teachers’ supervisory efforts and students’ MVPA levels during fitness instruction. The importance of active supervision, within die context of holding students accountable for engaging in physical activity, is underscored by this study.