Conservation of forests and stands should be based on a knowledge of the diversity of all their living components. Microfungi are a heterotrophic component important in maintaining the ecological balance of forests. Saprotrophic fungi, together with other micro-organisms, decompose wood; pathogenic fungi cause diseases of twigs and stems, desiccation of branches and dieback of trees; microfungal diversity can be affected, particularly in disturbed forests. The diversity of wood-inhabiting microfungi was examined in the protected ecosystem of the Carpathian Biosphere Reserve. The fungi, mainly non-lichenized ascomycetes and their anamorphs were studied from 1992 to 1995 in various forests of the reserve (Uholka, Kuziy, Stuzhytsia, Chornohora, Shyrokoluzhansky and Marmaroskyi massifs). Species were identified by inspection of trees, fallen and attached stems and branches, trunks and stumps, and by further examination of fruitbodies in the laboratory. Some bibliographic data about earlier collections in the reserve (Smits'ka, 1955) were also considered. In all 89 species (57 ascomycetes and 32 conidial fungi) of microfungi were collected. Among teleomorphs, pyrenomycetes predominated, with 60% of the total number. Of the 10 ascomycete orders encountered, the most species rich were Diaporthales (16 species), Xylariales (13) and Rhytismatales (8). The most species rich genera were Nectria (6 species), Diaporthe (5) and Hypoxylon (4). Among anamorphs, coelomycetes predominated. The most commonly observed and abundant wood-inhabiting ascomycetes in the reserve were Hypoxylon fragiforme on Fagus sylvatica; Diatrypella favacea on Fagus sylvatica, Quercus robur and Betula pendula; Quaternaria quaternata (and its anamorph Libertella faginea) on Fagus sylvatica. In addition to these, certain other microfungi were widespread, particularly in deciduous forests of the reserve: Diatrype disciformis, Diatrype stigma, Diaporthe eres and Valsa ambiens, for example, were ubiquitous common saprotrophs decomposing wood of various trees. Other species seemed, however, to be partially biotrophic, and pathogenic on trees weakened by other factors. These included Diatrypella favacea, Quaternaria quaternata (and its anamorph) and Nectria ditissima all on dying beech twigs, and Massaria inquinans, Diaporthe carpini and Libertella betulina causing necroses of maple, hornbeam and birch twigs respectively. Comparison of the present results with data on fungi of the reserve published in the 1950s reveals an increase in the number of bark pathogens. This may indicate ecological deterioration of these forests during the last 40 years.