The 76 isolates of Bipolaris maydis were obtained from diseased corn leaves collected at 35 sites in Japan and their mating type were determined by pairing with tester isokates, as in the case of the 4 isolates obtained from Panicum spp. and Bothriochloa sp. All the isolates were equally pathogenic to the differential lines of corn with T-cms, C-cms and N-c type cytoplasms in spore-spraying tests. They did not cause wilting of leaves in any type of cytoplasms in toxin tests and no host-specific toxins were detected. As a result, all the Japanese isolates were assigned to race O. Although the 75 isolates from corn produced only small and chlorotic lesions in the rhm resistant corn line, one isolate caused severe symptoms with typical and large lesions. The increased ability to cause lesion enlargement of the isolate in the rhm resistant line was attributed to the increase in the number of appressoria formed at the time of spore germination and faster hyphal extension in the host cells. This is the first report on a B. maydis isolate virulent to the rhm corn line in the world.