Prerequisite courses are essential tools in many university curricula, but some educators have suggested that they may be detrimental if applied too stringently to exclude students from courses. A measure of the value of prerequisites is whether or not they promote student success. The strongest way to test the value of prerequisites is to compare the success rate of students with and without prerequisites in higher-level courses. Although such comparisons can be based on instructor assessment of preparedness or even on student self-assessment, the most meaningful comparisons of student success are grades and withdrawal rates of students with and without prerequisites. This article presents the results of a comparison of the success rates of students with and without prerequisites in higher-level biology courses at the University of South Florida. Based on this study: (1) the faculty-driven model of the function of prerequisites yielded better course performance than the student-driven model; and (2) both failure and withdrawal rates of students in Department of Biology courses at dropped significantly when prerequisites were enforced. (Contains 1 figure.)