We investigated the root endophytic fungi of lupine using four approaches: (1) occurrence of fungal colonization in field-collected roots; (2) growth response of L. latifolius to inoculation with two types of fungi; (3) structure of root colonizations of Pinus and Lupinus by Phialocephala fortinii, a septate endophytic fungus of lupine; and (4) comparison root morphology, mycorrhizal colonization and natural ¹⁵5N-abundance N₂ fixation of three legumes. In part 1, three species of Lupinus were never observed to have fungal colonization; nine species were colonized by VA mycorrhizal fungi; seven species were colonized by fungi with septate hyphae which often formed intracellular scierotia, here called septate endophytes. In part 2, shoot weight of 16 week old L. latifolius seedlings in the greenhouse was significantly reduced by Glomus spp. in one experiment; p. fortinii significantly increased nodule weight in one experiment and reduced it in the other. In part 3, P. fortinii colonized root epidermal and cortical cells in the root hair zone on ultimate lateral pine roots, as well as cortical and epidermal cells of primary roots of Pinus and Lupinus. Fungal colonization was inter- and intracellular with scierotia forming in cells of both hosts. Labyrinthine tissue, a type of fungal differentiation which occurs in the Hartig net of ectomycorrhizae, formed sporadically on pine roots. In part four, Roots of Lupinus albicaulis cv. hederma had a significantly larger proportion of coarse roots (> 1 mm diameter) and significantly less mycorrhizal colonization than two other legumes, Medicago lupinula and Trifolium hybridum. Estimated aboveground N derived from fixation ranged from 6.1 to 39.9 kg per hectare (average = 22.0 kg/ha) and did not vary significantly among species. / Graduation date: 1992