The purpose of the study was to determine the effect of problem-based learning (PBL) on critical thinking ability and content knowledge among selected secondary agriculture students in Missouri. The study employed a quasi-experimental, non-equivalent comparison group design. The treatment consisted of two instructional strategies: problem-based learning and supervised study. The target population was identified as secondary agriculture students in Missouri. Twelve secondary agriculture teachers were selected based on criteria established by the researcher. Intact classrooms were randomly assigned to a level of the treatment. The resulting sample (n = 140) consisted of 77 students in the problem-based learning treatment group and 63 students in the supervised study treatment group. Analysis of covariance indicated a treatment effect on critical thinking ability and content knowledge. (Contains 5 tables.)