Six races of Plasmodiophora brassicae Wor. have been distinguished on the basis of pathogenicity on differential hosts of cultivated and wild crucifers.Pathogenic differences between isolates were established by replicated tests in the greenhouse under conditions of controlled temperature and moisture. In order to ensure adequate infection, the spore density of each inoculum was standardized before the seedlings were inoculated.Crucifers found suitable for the differentiation of races were: Wilhelmsburger swede, Nappan hybrid swede lines, Laurentian swede, Sisymbrium altissimum, Brassica nigra, Brassica campestris, and Wisconsin clubroot-resistant cabbage lines. Reactions in swede and cabbage were sharply differential. Wilhelmsburger swede proved resistant to all inocula except those of race 1. Certain Nappan hybrid swede lines were moderately resistant to race 1 and highly resistant to the remaining races. Laurentian swede showed resistance to races 5 and 6 only. Wisconsin cabbage showed marked resistance to races 5 and 6 and high susceptibility to races 2 and 4.Races 2, 3, and 4 were distinguished by reactions of B. campestris and S. altissimum, whereas race 5 was separated from race 6 by the greater susceptibility of B. nigra and B. campestris to race 5.