The study investigated the effects of full-credit semester and all-year timetables on science attitudes and science achievement of grade-10 students in British Columbia. All grade-10 students in British Columbia completed multiple matrix sampled assessment instruments in May of 1986. These instruments provided background information, affective scores, and cognitive scores which were used to compare the groups. It was found that, contrary to reported teacher perceptions of semester versus all-year courses, students in the all-year courses consistently outperformed both first- and second-semester students in the cognitive domains tested, and there were no significant differences in the affective domains. The finding that second-semester students out-performed the first-semester students casts doubt on the reported teacher perception that knowledge retention is of little concern under a semester system.