The expression of 12 cDNAs from Plasmodiophora brassicae, among them two novel sequences, was determined during clubroot development on Arabidopsis thaliana. The aim was to find cDNAs expressed at distinct stages of pathogenesis. The relative amount of infection with active plasmodia could be estimated using PbActin cDNA as an internal standard. Two cDNAs, PbBrip9 and PbCC249, were strongly expressed at stages of disease development corresponding to the occurrence of sporulating plasmodia. Therefore, it should be possible in the future to find more cDNAs which could be used as markers for certain stages of clubroot development.