A search for double-stranded RNA (dsRNA) was conducted among 106 isolates of the pathogenic basidiomycete Heterobasidion annosum. Of these isolates, 47 were tissue isolates from fruit bodies and 59 were isolated from decayed wood. Nucleic acids were extracted from freeze-dried mycelia and dsRNA was separated by cellulose CF-11 chromatography and confirmed by digestions with specific nucleases. dsRNA was present in 19 and 14% of the tissue and wood isolates, respectively. From five of the fruit bodies containing dsRNA basidiospores were investigated and 10-84% of the germinated basidiospores contained dsRNA. On high nutrient media, the germination frequency of basidiospores was reduced by presence of dsRNA in the fruit body (P < 0.05); germination frequencies were 34 and 78% for spores from fruit bodies with and without dsRNA, respectively. The same trend was present also on low nutrient media, although not statistically significant; germination was 3 and 10% for spores from infected and dsRNA free fruit bodies, respectively. Transmission of dsRNA in H. annosum from mycelia into basidiospores together with the lowered germination frequency are likely to play a significant role in the life cycle of the fungus. The relative importance of different transmission routes for dsRNA in H. annosum is discussed.