Intercellular and intracellular penetration of hyphae of Rhizoctonia solani into young, functional root nodules of soybean Glycine max inoculated with Rhizobium japonicum was restricted to the nodule's outer cortex. Penetration of the nodule central tissue may have been impeded by a layer of thick-walled sclerenchymatous cells. A remote cytopathogenic effect on Rhizobium-containing cells of the central tissue of young nodules was nuclear breakdown characterized by absence of stainable matrix material within the nucleus, dissolution or rupture of the nuclear membrane, extrusion of the nucleolus, and eventually lack of identifiable nucleus. A weak reactivity of diseased nodule tissue to benzidine-H 2O 2 at pH 4·5 indicated low leghemoglobin content. The degenerative effects did not occur in cells of young nodules grown in absence of the fungus. Normal cell degeneration in senescent nodules was primarily characterized by cytoplasmic and nuclear shrinkage without rupture of the nuclear envelope and thus differed from the cytopathogenic effect. These results support the concept that cell dysfunction in young nodules grown in the presence of R. solani may have been caused by toxic fungal metabolites that diffuse throughout the nodule. Fungal-induced nodule cell dysfunction would interfere with nitrogenase and symbiotic N 2 fixation activities and, where the N supply is sub-optimal, would reduce the yield of the soybean plant.