The objectives were to determine, with respect to Douglas-fir and ponderosa pine: (1) the frequency of occurrence of the fungal endophytes Byssochlamys nivea Westling and Umbelopsis spp. in woody roots, (2) the relationship between host size (tree diameter at breast height, DBH) and occurrence of these two endophytes, (3) the distribution of the fungal endophytes along the roots and in the rhizosphere around woody roots, (4) whether the fungal endophytes in woody roots are horizontally or vertically transmitted, (5) the in vitro minimum, maximum, and optimum growth temperatures of the fungal endophytes and, (6) whether Byssochlamys nivea is antagonistic in vitro against the pathogenic species Armillaria ostoyae Romagn., a common root pathogen.