Vegetative hyphae of Fusarium oxysporum differentiate into chlamydospore by triggering with carbon-starvation. The current changes in the cellular detoxifying defenses against superoxide and hydrogen peroxide: superoxide dismutase (SOD) and catalase, were examined. Although there was a little change in catalase, a dramatic change in SOD was observed during the differentiation. In vegetative hyphae of F. oxysporum f. sp. raphani, three isozymes of SOD, all of which were not inhibited by hydrogen peroxide and cyanide, were present whereas in chlamydospore an isoenzyme, which was inhibited by hydrogen peroxide but not by cyanide, was present. Thus, as differentiation proceeded, Mn-type SOD disappeared and an Fe-type SOD appeared. The results suggest that the Fe-type SOD is specifically expressed during chlamydospore formation and that active intermediates of oxygen and/or its scavenging enzymes participate in the differentiation of Fusarium oxysporum.