The main aim of this thesis has been to elucidate the species composition and community structure of ectomycorrhizal fungi associated with mature trees and naturally regenerated seedlings in natural boreal forests in Sweden. Further, the effects of disturbances, such as wildfire and nitrogen inputs, were studied. The thesis is based on 5 research papers, 2 published (Canadian Journal of Botany (1997) 75 (2) 1323-1335; New Phytologist (1997) 136 (2) 313-325) and 3 in manuscript form, all in collaboration with other authors; these are included. Sporocarp surveys, morphological stratification and DNA-based analyses of mycorrhizas were used to describe the mycorrhizal fungal communities. In addition, an ITS (Internal Transcribed Spacer region of rDNA)-RFLP (Restriction Fragment Length Polymorphism) reference database useful for identifying individual mycorrhizas was developed based on DNA analyses of sporocarp tissue. Overall, the species richness of ectomycorrhizal fungi was at least 30-40 times higher than that of their host trees. Naturally regenerated seedlings were colonized by the ectomycorrhizal fungal species present in the mycelial network of the old trees, indicating that the species composition will remain about the same provided that the host does not disappear. Wildfire, disturbing the fungal continuum, caused a shift in the frequencies of ectomycorrhizal fungi rather than a change in species composition. Nitrogen addition did not have any detectable effect on the abundance or species richness of mycorrhizas, but led to a decrease in sporocarp production. In all the studies, there was little resemblance between the species composition of sporocarps and that of mycorrhizas. The ITS-RFLP reference database was very useful in identifying single mycorrhizas, and proved to be a powerful tool for species identification of unknown mycorrhizas.