In this study, we investigated the diversity and ecology of Armillaria species in virgin pure beech and mixed conifer forests (15,000 ha) of the Carpathian Biosphere Reserve in Ukraine. Armillaria rhizomorphs were systematically sampled, both from the soil and from the root collar of trees (epiphytic), on 79 plots (25 x 20 m) of a 1.5 x 1.5 km grid. In both forest massifs, rhizomorphs were present in the majority of the soil samples, with an estimated dry weight of 512 kg/ha in the pure beech forests and 223 kg/ha in the mixed conifer forests. Similarly, in both forest massifs, most of the trees inspected had rhizomorphs at the root collar. Species identification based on DNA analyses showed that all five annulated European Armillaria species occur in these virgin forests, as previously observed in managed forests in central Europe. However, differences in the frequencies of the single species were observed. The predominance of the preferentially saprotrophic A. cepistipes and A. gallica (84 and 15% of the specimens, respectively) and the absence of significant pathogenic activity suggest that in these virgin forests Armillaria species are most likely to behave as saprotrophs. Forest management may increase the frequency of the pathogenic species A. ostoyae, which is rare in virgin forests.