Pathogenicity tests on potato stems were made of 133 isolates of Rhizoctonia Solani Kühn. Of these, 114 were from random sclerotia on random tubers from four fields, 13 from lesions on potato stems, and 8 from single basidiospores. A number of tests were made in the laboratory at 17° and 23 °C., in two contrasting types of artificially infested, unsterilized, virgin soil, which was maintained at optimum moisture content for disease expression.More of the isolates were pathogenic in the infertile podsol soil than in the fertile black loam. Eighteen per cent of the isolates were of virulent rank in the latter soil, in contrast to 34% of them in the former one.Indications from the study were that, under average soil conditions, approximately 20 to 50% of the isolates of R. Solani from sclerotia on random tubers may be assigned to the zero and marginal classes of pathogenic rank. The data also indicated that certain isolates were inherently very deficient in pathogenicity to potato stems, while others characteristically possess a high degree of virulence. Thus, with regard to the effect of soil type and racial differences in pathogenicity, it would appear that the results of this study help to explain why the stems of a high percentage of plants from sclerotia-infested sets often escape with little or no infection under field conditions.