Alterations of phenolics in potato tuber discs treated with coronatine, a pathotoxin produced by Pseudomonas syringae pv. atropurpurea, were examined in relation to the changes in some oxidase activities involved in their metabolism. The content of phenolics was higher in coronatine treated discs than in non-treated discs. This was also true for the levels of polyphenol oxidase, peroxidase and ascorbate oxidase activities, their increases by coronatine treatment being markedly inhibited by cycloheximide or actinomycin D. An isoperoxidase induced specifically by coronatine treatment was recognized by isoelectric focusing electrophoresis. Among substrates tested, only o-diphenols were oxidized by polyphenol oxidase, and o- and p-diphenols (except guaiacol) by peroxidase irrespective of treatment. Researches on the physiological activities induced by coronatine were discussed to be useful for the elucidation of host-parasite interaction.