N-(2, 6-Dialkylphenyl)-4 (or 3)-carboxyphthalimides and the related compounds were prepared and their preventive activity against clubroot of Chinese cabbage caused by Plasmodiophora brassicae was examined. Among them, N-(2,6-diisopropylphenyl)-and N-(2-sbutyl-6-ethylphenyl)-4-carboxyphthalimides were the most active. The preventive activity of the corresponding esters and amides varied significantly with variation of the alcohol and amine moieties. Relationships between the chemical structure and the preventive activity are described.