The effects of All Stars, a character education and problem behavior prevention program, on variables theorized to mediate problem behaviors and on the problem behavior variables of substance use, sexual behavior, and violence among middle school students are reported. In an independent, randomized, single-cohort, longitudinal evaluation of the program, 1,655 students completed pretest, posttest, and 1-year follow-up surveys measuring demographics, mediating variables, and behavioral outcome variables. Results indicate that the All Stars program, when administered by teachers, had an immediate effect on mediating variables that did not persist over time. Inclusion of ethnicity in the design showed that the program, when administered by specialists, had delayed effects on mediating variables for African American and Hispanic students. However, no consistent effects were found for student problem behaviors in either condition. Implications for prevention practice and directions for future research are discussed.