In previous studies, researchers have identified a general low level of health-related fitness (HRF) knowledge among secondary students that can effect levels of physical activity (PA). An instructional strategy that may increase HRF knowledge without decreasing PA is the personalized system of instruction (PSI). Two classes 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. Group 1 (n = 24) completed a unit on personal fitness using the PSI model, and Group 2 (n = 29) used a traditional DI approach. Knowledge was assessed 3 times (pre, post, 3-week follow-up) using a 45-question standardized HRF knowledge test. Class time PA was reported using a modification of the SOFIT observation system. A 2 × 3 ANOVA was used to compare HRF knowledge scores, showing a significant increase in Group 1 scores from pre- to posttest (p= 0.003) as well as significant increases in scores between Group 1 and Group 2 (p = 0.03). Physical activity results were compared using a pair-samples t test with outcomes revealing differences in class time PA levels (t = -0.27, p = 0.79). These results indicate PSI is a successful model for increasing HRF knowledge while maintaining physical activity levels.