Teaching fitness-related knowledge has become critical in developing children's healthful living behavior. The purpose of this study was to examine the effects of a science-based, constructivist physical education curriculum on learning fitness knowledge critical to healthful living in elementary school students. The schools (N = 30) were randomly selected from one of the largest school districts in the United States and randomly assigned to treatment curriculum and control conditions. Students in third, fourth, and fifth grade (N = 5,717) were pre- and posttested on a standardized knowledge test on exercise principles and benefits in cardiorespiratory health, muscular capacity, and healthful nutrition and body flexibility. The results indicated that children in the treatment curriculum condition learned at a faster rate than their counterparts in the control condition. The results suggest that the constructivist curriculum is capable of inducing superior knowledge gain in third-, fourth-, and fifth-grade children. (Contains 4 tables and 1 note.)