Two isolates of raspberry bushy dwarf virus (RBDV) from black raspberry (Rubus occidentalis) originating from the USA were distinguishable from isolates from red raspberry (Rubus idaeus) by spur formation in gel-diffusion serological tests. Studies with one black raspberry isolate showed that it resembled strains from red raspberry in host range and symptomatology but was more difficult to transmit manually and differed in properties in vitro. In Chenopodium quinoa sap its dilution end-point was 10-1-10-2, thermal inactivation point 45-55C and longevity in vitro 2 h, compared with values of 10-3-10-4, 55-65C and 3-4 days for a red raspberry isolate. The protein (mol.wt 29 000) and RNA (three components, mol.wt 2.0, 0.8 and 0.3 x 106) composition was similar to that of a red raspberry isolate. All red raspberry isolates from Europe and North America that have been studied are serologically identical to each other but it is not yet known whether the same is true of black raspberry isolates. It seems possible that species-specific strains could have arisen as a result of the mode of transmission by pollination.