Similarity of organisms encountered in studies of black root of strawberry and of tobacco, respectively, carried out contemporaneously but independently, suggested the co-operative investigation, the results of which are embodied in this paper. Strawberry and tobacco seedlings growing (i) in seed-bed muck heavily infested with Thielaviopsis basicola (Berk.) Ferraris, and other organisms known to be pathogenic on tobacco, (ii) in soil from a commercial plantation where strawberry root rot had occurred in severe and typical form, and (iii) in greenhouse compost soil, were examined microscopically daily, commencing a few hours after germination and continuing throughout a period of four weeks. Organisms observed definitely within root tissues of both hosts included the "phycomycetous mycorrhizal" fungus, T. basicola (observed in plants grown in muck only), Rhizoctonia (Solani and endophytic orchid types), forms of Pythium, Asterocystis (Olpidiaster), certain unidentified fungi, a minute filamentous alga and nematodes. Organisms observed on the surface of roots included representatives of the genera Cylindrocarpon (Ramularia), Fusarium, Helminthosporium, Sphaeropsis, and Cephalothecium. The sequence of appearance, percentage occurrence, and parasitic capabilities of certain of the organisms varied in roots grown in the different soils. Because of early infection by, and ultimate almost universal occurrence of, the phycomycetous mycorrhizal fungus this organism received especial attention. Evidence based on certain morphological differences suggests the occurrence of strains of this organism. Of interest, too, is an alga invading living root tissue.From observations not limited alone to the examination of diseased roots of strawberry and tobacco, the authors are led to conclude, (1) that a root rot as it occurs in nature is extremely complex even in cases where a primary causal agent is recognized, and (2) that fungi representative of comparatively few groups or genera are "common factors" in root-rot complexes of different host plants.The technique described offers distinct advantages in that it permits a study of the sequence and severity of infection by the organisms involved in a root-rot complex; it reveals the occurrence of obligate parasites the presence of which would never be detected by the soil-plating, the Cholodny, or the tissue-isolation methods; and it is readily adaptable to the study of other root-rot complexes.