Seedlings of Pseudotsuga menziesii (Mirb.) Franco, Pinus ponderosa Dougl. ex Laws, Arbutus menziesii Pursh., and cuttings of Arctostaphylos uva-ursi (L.) Spreng were grown in monoculture and in conifer-hardwood dual-culture combinations in the glasshouse and inoculated with spore slurries of six Rhizopogon species. The primary objectives were to assess and compare the pattern of host specificity between symbionts and to study the influence of co-cultured plants on ectomycorrhiza development. The Rhizopogon spp. ranged from genus-specific to multiple-host compatible. In monoculture, four Rhizopogon sp. (R. ellenae Smith, R. occidentalis Zeller & Dodge, R. smithii Hosford and R. subcaerulescens Smith) formed ectomycorrhizas with Pinus ponderosa, and two Rhizopogon sp. (R. parksii Smith and R. vinicolor Smith) formed ectomycorrhizas with Pseudotsuga menziesii. None of the fungi tested developed ectomycorrhizas on Arbutus menziesii or Arctostaphylos uva-ursi in monoculture. In dual culture, three of the four Rhizopogon species (R. ellenae, R. occidentalis and R. subcaerulescens) that formed ectomycorrhizas on Pinus ponderosa, formed some ectomycorrhizas on Arbutus menziesii and Arctostaphylos uva-ursi. Rhizopogon parksii and R. vinicolor only formed ectomycorrhizas on Pseudotsuga menziesii.