Commitment to the principles of sportspersonship is an acknowledged goal for school physical education. However, few programs have been implemented to investigate moral development changes in physical activity settings. A field experiment was designed to examine the effect of participation in educational activities selected from Fair Play for Kids (1990) on the moral judgment, reason, intention, and prosocial behavior of children (N = 452) in the 4th through 6th grades. Six intact classrooms at each grade level (N = 18) were randomly assigned to the following groups: (a) control, (b) Fair Play for Kids curriculum during physical education only, or (c) Fair Play for Kids curriculum during all school subjects. Experimental protocol extended for 7 months of an academic year, and moral development indicators were assessed prior to and following the intervention. Using class as the unit of analysis, 3 x 2 (Group x Time) repeated measures analyses of variance revealed that both treatment groups were significantly higher than the control group at posttest for moral judgment, reason, and intention scores. For students within classes, repeated measures analyses showed that treatment group participants had significantly higher posttest scores on all 4 measures as compared to students in the control group. Results provide initial validation of the Fair Play For Kids curriculum for effecting change in the moral development of elementary school students.