The plant pathogenic basidiomycete, Rhizoctonia solani, contains extrachromosomal double-stranded RNA and DNA elements, but the role of these elements in the biology and pathology of the fungus is uncertain. Aspects of these elements in R.solani and the role of anastomosis (hyphal fusions) in their transmission are examined here. Anastomoses between hyphae, leading to successful cell fusions and death of fused cells (vegetative incompatibility) were observed by video microscopy and by fluorescence microscopy when hyphae were loaded with fluorochromes. However, attempts to monitor organelle transfer were unsuccessful and ultra-violet irradiation of hyphae containing fluorochromes led rapidly to hyphal death. Two strains of anastomosis group (AG) 4 could readily be 'cured' of dsRNA by subculture of hyphal tips, although one strain which contained a 2.5kb DNA element could not be freed in this way, nor by ultra-violet irradiation or heating to 30°C. Several of the resulting hyphal tip subcultures showed an incompatibility reaction when paired with the respective parent strain. These parent-incompatible strains (6 from parent strain PA1 and 6 from parent strain I13) fell into 2 groups - mutually compatible within each group, but incompatible with the other group and the parent. Anastomosis during pairings of strains within any one group never led to a parent-compatible strain when subcultures were taken from the zone of hyphal fusion. There was no evidence that dsRNA influenced compatibility; instead it is suggested that hyphal tip subculturing led to segregation (or expression) of nuclear compatibility genes. Counts of nuclei in tip cells, by DAPI staining and fluorescence microscopy, showed variation in different parts of the fungal colonies, and significant tendency for some juxtaposed branch tips (arising as clusters from a single hypha) to have similar nuclear numbers to one another.