The infection of Arabidopsis thaliana with the causal agent of the clubroot disease, Plasmodiophora brassicae Wor., leads to tumorous swellings of the root as a result of increased cell division and hypertrophy. Recently, it was shown that nitrilase 1 and 2 were prominently enhanced in clubroots compared to controls. To investigate the role of nitrilase for symptom development, Arabinopsis thaliana plants transformed with nitrilase 1 and 2 in antisense direction (aNIT1, aNIT2) were infected with P. brassicae. The infection rate of aNIT1 plants was reduced compared to the wild type, whereas the infection rate was unaffected in aNIT2 plants. The size of root galls was reduced in aNIT1 plants throughout club development, whereas aNIT2 planes were only delayed in club development and showed similar club weight as the wild type 42 days after inoculation. A smaller number of fungal structures was observed in aNIT1 and aNIT2 plants and the resting spores were not clustered within large sporangia compared to a typical clubroot infection. The reduced fresh weights correlated with reduced nitrilase mRNA and protein, thus resulting in lower free IAA content in the smaller clubs of the antisense plants.