SUMMARYThe soil microfungi in five northern Wisconsin open bogs and ten conifer swamps have been surveyed. Two hundred isolates from eight sites in each community were identified or characterized and frequencies for all entities were calculated. The number of entities per community ranged from 10 to 68, and the numbers of entities per unit (1000 isolates) were 108 (open bogs), 104 (Picea-Larixswamps), and 181 (Thuja-Abiesswamps). Only 5–9 species in each unit had average frequencies of 30% or higher. Mycelia Sterilia 3157, Eurotiaceae gen. n.? 4084,Penicillium odoratum, Penicillium (Paecilomyces ?)sp. 3908, andGeotrichum candidumare prevalent in open bogs;Mortierella vinaceaIII,Periconiasp. 3284,Penicillium odoratum, Cryptococcus laurentii, Oidiodendron flavumvariant, and one unidentified yeast inPicea-Larixswamps; andTorulopsis aeria, Cryptococcus laurentii, Trichoderma viride, T. album, and one unidentified yeast inThuja-Abiesswamps. In the populations as a whole, Ascomycetes, yeasts, Dematiaceae, andMortierellaspecies were common, whereasAspergillus, Fusarium, and Sphaeropsidales were rare. Approximately one-half of the identified species appear to be absent or rare in mineral soils, but several are well-known wood, wood-pulp, or litter forms. Species composition in the microfungal communities is correlated with species composition and maturity in the overlying higher plant community.