Abstract: One hundred and thirteen isolates of Bipolaris maydis,collected from 13 prefectures including 42 counties or cities in Yunnan,were identified based on their pathogenic types on the Chinese Differentials.The results showed that 4 physiological races O,S,Tand Cexisted in Yunnan,and among them Race Owas dominant and had different pathogenicities.Medium and weak pathogenic isolates of Race Owas significantly morethan those of the strong one.Besides,there was discrepancy about distribution of physiological races of B.maydis at different altitudes,especially Race Sdistributed at high altitude.