Twenty nine isolates of Rhizopogon from 20 spp. differed markedly in their ability to form ectomycorrhizas with Douglas fir, western hemlock and lodgepole pine in pure-culture syntheses, Within the four sections of Rhizopogon there were similarities in ectomycorrhizal appearance and range of potential hosts. Intermediate and overlapping ectomycorrhiza! characters were evident in closely related species complexes. Although the host-specificity of many species, based on field observations of sporocarps, often correlated with their host range in the syntheses, some fungi formed well-developed ectomycorrhiza on hosts with which they appear not to be associated in the field. The specificity of host associations is suggested as a major contributor to the speciation and diversification of Rhtzopogon in the Pacific Northwest of the USA and Canada.