The purpose of this study was to determine students' health-related fitness knowledge (HRFK) and physical activity levels after the implementation of a series of fitness lessons segments called Knowledge in Action (KIA). KIA aims to teach health-related fitness knowledge (HRFK) during short episodes of the physical education lesson. Teacher participants from one district (N=10) were randomly assigned into either the intervention or comparison group. Intervention teachers used the KIA fitness lessons during fifth grade students' physical education classes. These teachers received training sessions, teaching materials, and YouTube videos that modeled the KIA fitness lessons. Intervention fidelity was assessed through observations and a fidelity checklist. Students' physical activity levels were measured using accelerometers and HRFK was examined by PE Metrics 28-question pencil and paper test. General linear models (GLM) and Hierarchical linear models (HLM) were used to examine group differences. Intervention students had a 3.4 (20%) greater unit improvement in HRFK scores when compared with their comparison counterparts (p<0.001), at school level. Student activity levels of moderatevigorous physical activity (MVPA) time were similar in both groups (p = 0.64). Teachers can use the KIA fitness lessons or similar strategies to effectively teach HRFK in elementary physical education classes.