Isolates of the snow mold fungus, Typhula ishikariensis, were collected from seven localities in northern Japan where snow cover conditions differ. The isolates were identified as T. ishikariensis biotype B, which produces large sclerotia, biotype C with smaller sclerotia, and large sclerotial and intermediate forms. Mating experiments with tester monokaryons indicated that all populations were related. Populations from localities with deep, persistent snow cover produced larger sclerotia, which germinated more readily carpogenically (biotype B and the large sclerotial form) than those from localities where snow cover was shorter and intermittent (biotype C). The aggressiveness of the former was more variable, whereas the latter was without exception aggressive. There was no apparent correlation between growth rate at 0 °C and duration of snow cover. It was concluded that biotype C has been selected for localities with ephemeral snow cover, but that biotypes B and C and the related forms do not form distinct populations and should be regarded as a single taxon.