The ectomycorrhizal communities of three aspen-covered sites in southwestern Montana and southeastern Idaho with varying stand age, conditions, and soil types were compared. In all, 39 species of ectomycorrhizal fungi were associated with P. tremuloides. Dominant fungal species varied by site. Species reported on acidic soils in Europe and other species reported to be "early colonizers" were found exclusively on the acidic soils of the Butte site. Many "late stage" fungi such as Amanitas and Cortinarii were found on the older and more productive sites. Nine isolates of ectomycorrhizal fungi were tested for their ability to form mycorrhizae with P. tremuloides. Amanita muscaria v. formosa, A. pantherina, Paxillus vernalis, and Pisolithus tinctorius formed mantles and Hartig nets. Inocybe lacera and Piloderma croceum formed mantles but no Hartig nets. Dry weight, stem diameter, height and number of roottips increased in the young aspen seedlings inoculated with a majority of the fungi listed above. The implications for aspen survival and growth are discussed. / Master of Science