All isolates of Rhizoctonia solani (teleomorph Thanatephorus cucumeris) anastomosis group (AG) 3 from potato tubers in commercial fields in Scotland were found to contain more than one double-stranded RNA (dsRNA) element ranging in size from 0·3 to 15 kb. Strains of other AGs (2-1, 3, 4, 5) from culture collections also contained dsRNA elements. Isolates of AG 3 from different black scurf lesions on single tubers showed similar dsRNA profiles and, when opposed on agar plates, underwent compatible hyphal fusion where the colonies met. Isolates from different tubers in the same commercial field often showed somatic incompatibility (mean 44%) leading to cytoplasmic death when colonies fused. Isolates from different fields always showed somatic incompatibility after hyphal fusion, suggesting that the complex population structure of AG 3 could significantly limit the transmission of dsRNA between strains. The incidence of different dsRNA elements in AG 3 isolates could not be related to pathogenicity in seedling tests on tomato, carrot, lettuce, radish, cress or onion. Double-stranded RNA elements could not be eliminated from AG 3 isolates by repeated hyphal-tip subculture or extended (8 week) incubation on agar containing cycloheximide. Some isolates lost the smaller dsRNA elements (1·0 to 1·6 kb) during these procedures, and one isolate (of AG 4) lost a 6·5 kb dsRNA element, but the larger elements (up to 9·5 or 12 kb) were always retained. When assessed by low stringency hybridisation, the dsRNA elements of R. solani AG 3 and of strains from AGs 2-1, 4 and 5 showed no homology with a plasmid containing cDNA of the complete hypovirus dsRNA genome of Cryphonectria parasitica.