The investigations reported here have centred largely around isolations from, and critical microscopic examination of, the roots of affected plants. Isolations from 684 young, adventitious and lateral roots showing definite lesions bordered by turgid, healthy tissue, yielded representatives of approximately 20 different genera of fungi, ranging in frequency of occurrence from 0.4% in the case of Gliocladium to 32.7% in the case of Fusarium. Next to Fusarium, isolates of Ramularia and Pythium appeared most frequently and consistently in culture, the former in 28.5%, and the latter in 10.8%, of the plantings. Results of infection experiments involving these as well as other genera, including Penicillium, Rhizoctonia, Hainesia, Cylindrocladium, Coniothyrium and Helminthosporium, are not yet available and it is impossible at the moment to evaluate the significance of their association with the diseased condition of the roots from which they were isolated. Isolations from 125 apparently healthy rootlets occurring on root systems, other parts of which were more or less severely diseased, yielded representatives of nine different genera of fungi, ranging in frequency of occurrence from 1.6% in the case of Alternaria to 5.6% in the case of Ramularia. Isolates of Fusarium appeared in 4.0% of the plantings.Microscopic examination of 550 adventitious and lateral roots of wild and cultivated plants has revealed the almost universal occurrence in their tissues of two of the so-called "endotrophic mycorrhizal fungi." One of these is of the characteristic phycomycetoid type, producing arbuscules and vesicles, the other, of the Rhizoctonia type familiar in orchids. The former type has been observed much more consistently than the latter. In a number of instances both types have been found in the tissues of the same rootlet. At least three strains of the Rhizoctonia type have been isolated but so far all efforts to culture the other organism have been unsuccessful. Resting spores of Olpidiaster (Asterocystis) and the spherical, smooth-walled spores of some member of the Plasmodiophoraceae were observed in diseased rootlets in the early part of the growing season. Pythium was noted in abundance occurring on diseased roots obtained in the field during September and November.Nematodes have been encountered in association with diseased roots in numbers and frequency sufficient to suggest a possible causal relationship to strawberry root rot.