Microarray analysis was used to investigate changes in host gene expression during the primary stages of the interaction between the susceptible plant Arabidopsis thaliana (L.) Heynh ecotype Col-0 and the biotrophic pathogen Plasmodiophora brassicae Woronin. Analyses were conducted at 4, 7 and 10 days after inoculation (DAI) and revealed significant induction or suppression of a relatively low number of genes in a range of functional categories. At 4 DAI, there was induced expression of several genes known to be critical for pathogen recognition and signal transduction in other resistant hostpathogen interactions. As the pathogen further colonised root tissue and progressed through the primary plasmodium stage to production of zoosporangia at 7 and 10 DAI, respectively, fewer genes showed changes in expression. The microarray results were validated by examining a subset of induced genes at 4 DAI by quantitative real-time reverse transcriptase PCR (RT-qPCR) analysis all of which correlated positively with the microarray data. The two A. thaliana mutants jar1 and coiI tested were found to be susceptible to P. brassicae. The involvement of defence-related hormones in the interaction was further investigated and the findings indicate that addition of salicylic acid can suppress clubroot disease in A. thaliana plants.