||Levels of physical activity and health-related fitness (HRF) are decreasing among adolescents in the United States. Several interventions have been implemented to reverse this downtrend. Traditionally, physical educators incorporate a Direct Instruction (DI) strategy when teaching, potentially leading students to disengage during class. An instructional strategy that has been shown to be effective in increasing content knowledge and skill competency in physical education is the Personalized System of Instruction (PSI). A two-study approach was used to investigate: (a) the fidelity of using PSI to teach HRF content knowledge and resistance training skills in a high school physical education class, and (b) examine changes in HRF content knowledge and in-class physical activity levels with a class using PSI compared to a similar class using DI. Students (N=54) from a private, urban high school in a major city within the Mountain West region of the United States participated in the 6-week study. Video and audio taping, along with interviews and journals, were used to determine if criteria standards associated with PSI were met. Knowledge was assessed three times (pre, post, 3-week follow-up) using a standardized HRF knowledge test. Scores were compared between the two groups in addition to changes within each group. Class time physical activity was measured using a modification of the System for Observing Fitness Instruction Time (SOFIT). Study results showed that 3 of the 4 components of PSI were met as well as 10 of 12 design features, suggesting that implementing the personal fitness unit using PSI was successful. Knowledge results showed that the PSI group demonstrated a significant increase in knowledge assessment scores from pre- to posttest (p=0.003). Between-groups results showed that after the 6-week study, HRF knowledge scores from the PSI scores were significantly higher than those from the control group (p=0.03). Differences in physical activity between the two groups were not significant (p=0.79). These results suggest that PSI is a successful instructional model for increasing HRF knowledge while maintaining physical activity levels. With its characteristics of self-pacing and mastery learning, PSI has the potential to be an effective teaching model within physical education.