Sixty-seven isolates of the southern blight fungus from Japan were divided into five groups based on ITS-RFLP analysis of nuclear rDNA. Morphological characters of sclerotia varied between groups. Three groups were reidentified asSclerotium rolfsii, and two resembledS. delphinii in RFLP patterns and/or in having large sclerotia and relatively low optimal growth temperature (28°C). Sclerotia of the latter, however, varied in size according to temperature and became indistinguishable from those ofS. rolfsii at high temperatures. Hyphal anastomosis (imperfect fusion) was observed between different ITS-RFLP groups, as well as between different isolates belonging to the same groups. These results indicate that populations of this fungus in Japan consists of several different subgroups, although morphological differences are not always evident.