When taro tubers (Colocasia antiquorum Schott) were inoculated by a taro strain (compatible or pathogenic) or a sweet potato strain (incompatible or non-pathogenic) of Ceratocystis fimbriata, lipid peroxidation, which was measured by the thiobarubituric acid reaction, took place to a similar magnitude and with a similar course in both kinds of tubers. Throughout the incubation period, no generation of superoxide anions (O2-) due to inoculation by either strain was detected in the tubers. The activities of phospholipase A2 and lipoxygenase changed in a manner accounting for the production of lipid peroxides observed in taro tubers inoculated by both strains. The sweet potato and taro strains had similar sensitivity to the toxicity of peroxides of linolenic acid and lipid peroxides from inoculated taro tubers. These results suggested that lipid peroxide produced by the host plant was involved in the early host-parasite interaction in this system.