Root colonization patterns were studied after Phialocephala fortinii was inoculated on Lupinus latifolius (broad-leafed lupin), a nitrogen-fixing legume, and Pinus contorta (lodgepole pine). The fungus colonized epidermal and cortical cells in the root hair zone on ultimate pine roots, as well as cortical and epidermal cells of primary roots of both hosts. Fungal colonization was inter- and intracellular with sclerotia forming in cells of both hosts. Labyrinthine tissue, a type of fungal differentiation that occurs in the Hartig net of ectomycorrhizas, formed sporadically on pine roots. Similar colonization has been observed on conifers and many other plants, but this report is the first showing that a single fungus can form such structures on both pine and lupin.