Modes of action of benomyl, calcium cyanamide, quintozene, trichlamide (NK 483/WL105305) and EF 70513 for control of Plasmodiophora brassicae Wor. (clubroot) have been studied. Materials were selected because of known activity against P. brassicae and provided a range of chemical structures and requirements for field application. All experiments were made in the glasshouse using seedlings of Brassica campestris ssp. pekinensis cv. Granaat and chemical concentrations in the range 100-2000 mg a.i./kg compost or soil. The effects of chemicals on pathogen development, symptom severity, host growth, secondary infection and resting spore survival were quantified and residual activity of each chemical at times after application was assessed. Pathogen development was quantified up to 14 days after inoculation in terms of primary root hair invasion and the development of primary plasmodia, mature zoosporangia and partially and fully evacuated zoosporangia. Club development and severity were assessed up to 35 days after inoculation. All chemicals were associated with reductions in pathogen invasion and subsequent symptom development. Benomyl and trichlamide were most effective at inhibiting root hair infection and in reducing club development. Colonization from secondary zoospores was inhibited most effectively by benomyl. Residual activity was most apparent with trichlamide, followed by EF70513, benomyl, quintozene and calcium cyanamide. Only calcium cyanamide possessed fungicidal effect which increased in proportion to the interval between incorporation and planting. Germination of P. brassicae spores was inhibited by calcium cyanamide, EF70513 and trichlamide but only to a limited extent by benomyl and quintozene. Phytotoxicity was not apparent with trichlamide and quintozene; EF70513 produced severe effects at high concentrations, at which benomyl also reduced plant height. Calcium cyanamide was phytotoxic only when seedlings were planted within 7 days of application or when high concentrations were used.