The obligate biotrophic pathogen Plasmodiophora brassicae induces cataplastic galls in the roots of various species of the Brassicaceae. The ecotypes Ze-0, Tsu-0, and Ta-0 of Arabidopsis thaliana carry alleles of a dominant single gene (RPB1) for resistance against this pathogen. The pathotype-specific resistance reactions in the roots were accompanied by a hypersensitive response. Infected cells were surrounded by necrotic boundaries and thereby the pathogen growth was restricted. The pathogen could induce hyperplasia and hypertrophy to a slight extent and develop spores within mini-galls surrounded by the necrotic cells, if it was able to colonise meristematic tissue. Ecotype Ze-0 showed the highest level of resistance to P. brassicae isolate ‘e’ as indicated by the very limited number of infected cells, whereas the resistance of the ecotype Ta-0 was rated lower. The results are discussed in relation to the interaction of the pathogen with Brassica crops.