Curricula at most colleges and universities in the United States are scheduled according to quarters or semesters. While each schedule has several potential advantages over the other, it is unclear what effect each has on student performance. This study compares biology student performance during the two and a half years before and after the 1999 switch from quarters to semesters at the University of Minnesota-Twin Cities. Student performance was assessed by examining grade distributions, point totals, and scores on archived exam questions. The change from quarters to semesters resulted in a small but significant overall decrease in student performance as judged by mean final scores, grade distributions, and exam question scores. Because student education is the primary duty of colleges and universities, these results have important implications for any institution considering a change in its curricular schedule.