A dikaryotic resting spore or two monokaryotic resting spores of Plasmodiophora brassicae were inoculated onto 1-d-old seedlings of Brassica campestris. The relative DNA content of a nucleus in a dikaryotic resting spore corresponded with that of a haploid nucleus in a monokaryotic resting spore. Plasmodia and zoosporangia were observed within the galls in some root cortical cells approximately 2 mo after inoculation. Resting spores were also formed within the galls in some cortical cells, and root hair infection was observed. The frequency of resting spore formation in B. campestris seedlings inoculated with a dikaryotic resting spore or two monokaryotic resting spores was 26·2 and 3·6%, respectively. The results indicate that the life-cycle of P. brassicae was completed on plants infected with a dikaryotic resting spore or by two monokaryotic resting spores.