More than three hundred Phytophthora infestans isolates collected from potato crops in Japan from 1987 to 1989 were investigated for their cultural features and divided into two groups, designated as S and M types. On potato tuber slices the S type isolates formed thin and appressed colonies producing abundant sporangia, while the M type isolates formed cottony colonies producing few sporangia. On V-8 juice agar (V-8A) the S type isolates grew well, while fresh isolates of the M type could not grow well. Cultural features on tuber slices and V-8A often changed after subculturing and storage. On rye A agar both types of isolates always grew well. On oatmeal agar the S type isolates could not grow, while the M type isolates grew well, and the features were stable. The cultural types S and M coincided with the mating types Al and A2, respectively.