Out of 164 plants of clubroot-susceptible Chinese cabbage inoculated with single resting spores ofPlasmodiophora brassicae, two plants developed clubroot symptoms. The two single-spore isolates (SSIs) extracted from these plants gave an identical reaction pattern on the European Clubroot Differential set (ECD) and seven doubled-haploid lines (DH-lines). Their reaction pattern differed from that of the original field isolate on four hosts: ECD hosts 06 and 07 were susceptible to the field isolate but resistant to both SSIs, while for DH-lines Bi and Pt the reverse was true. DH-line Pt was significantly less diseased by mixed inocula consisting of the field isolate and SSI-1 than by SSI-1 alone. It was concluded that the SSI-1 pathotype was a minor component of the field isolate, although it was isolated twice. The results also suggest that the alleviating effect of the field isolate in mixed inoculations with SSI-1 on DH-line Pt was due to induced resistance, rather than to competitive interactions.