Two cultivars of rapeseed, Span (Brassica campestris, L.) and Zephyr (B. napus, L.) were inoculated at various intervals, beginning at 36 days after seeding, with Alternaria brassicae, A. raphani, or a combination of these fungal species. At harvest time plants were ranked into slightly, moderately and severely diseased categories. Seed from these plants was analyzed for yield, oil content and protein content. Yield reductions in plants with a severe incidence of alternaria blackspot, caused by different inoculation treatments, in Span and Zephyr, respectively, were: both species of Alternaria, 70 and 42%; A. brassicae, 63 and 42%; and, A. raphani, 42 and 34%. Reductions in kernel weight of severely diseased plants were significant, but accounted only partially for yield reductions. Significant reductions in oil content occurred in Span in all severely diseased plants, but there were no significant reductions in oil content of Zephyr. Protein content of Span was reduced in all disease ranks of plants inoculated with A. brassicae. In Zephyr, reductions in protein content occurred in plants that were slightly or moderately affected by A. brassicae or both species. Thus, in most instances Span was affected more than Zephyr by alternaria blackspot.