PCR-RELP analysis of the ITS region in the ribosomal RNA gene cluster revealed that Japanese isolates of the southern blight pathogen consisted of 5 groups. Groups 1, 2 and 3 corresponded to Sclerolium rolfsii, whereas groups 4 and 5 were similar to S. delphinii, a related species of S. rolfsii. Most of the isolates belonging to groups and 2 were detected in the central to southwestern regions of Japan, while groups 3, 4 and 5 were distributed in the central to northern regions, Although the ITS-RFLP groups also differed in the size of the sclerotial, the sclerotial size varied with the incubation temperature and became indistinguishable at high temperatures. These groups showed hyphal anastomosis with one another, indicating their conspecificity.