Fungi, especially the higher basidiomycetes, differ from other organisms in having a particularly great variety of life forms within individual species. This variety can be observed not only among individuals but also within the life cycle of a single organism (through formation of a fruit body, for example). In contrast to insects, these changes are continuous, not in phases. The life cycle in higher basidiomycetes includes haploid, dicaryotic, and diploid stages, each of which may vary greatly. In most heterothallic species, basidiospores and the primary mycelium resulting from their germination comprise the haploid stage. Study of monokaryotic mycelium is interesting because in nature, for most fungi, genetically different mycelium behaves as an individual in terms of physiology and ecology (Rayner, 1991). Furthermore, mechanisms exist which maintain the separate existence of vegetative individuals but which permit hyphal fusion for sexual reproduction. The subjects of our investigation were monocaryotic cultures obtained from fruit bodies collected in nature (Coriolus versicolor, Oudemansiella mucida), or laboratory-grown (Flammulina velutipes, Sparassis laminosa). Monokaryons of all species examined revealed considerable variability in growth rate on agar, in rate of biomass accumulation and in wood-decomposing activity. The influence of mineral elements on growth of different monokaryons was noted, and the viability of different monokaryons stored in pure culture was ascertained. Physiological features of monokaryons and dikaryons obtained by dikaryon crossing were studied. Thus, monokaryotic cultures can be used to select fungi and to create a collection of rare and species in danger of extinction, to conserve their diversity.