A common tuberculate ectomycorrhiza of Douglas fir in the Pacific Northwest, described earlier by Trappe, is further examined and defined. Tubercles consist of an outer rind of aseptate, amber, thick-walled hyphae encasing tightly packed inner elements mantled with septate, hyaline, thin-walled hyphae. Reported as a Phycomycete and a Basidiomycete, respectively, the two hyphal forms actually belong to a single fungus,Rhizopogon vinicolorA. H. Smith; cultural characteristics of this fungus are described. Pure culture mycorrhiza syntheses with both mycorrhizal and sporocarpic isolates and Douglas-fir seedlings are reported. Antagonism tests revealed the following inhibition of root pathogens byR. vinicolor: strong—Phytophthora cinnamomiRands,Pythium debaryanumHeese, andPythium sylvaticumCampbell & Hendrix; moderate—Fomes annosus(Fr.) Cke. andPoria weiriiMurr.; and weak or none—Fusarium oxysporumf.pini(Hartig) Snyd. & Hans.,Pythium ultimumTrow,Rhizoctonia solaniKuehn, andMacrophomina phaseoli(Maubl.) Ashby.