Pronounced chlorotic line patterns and ringspots developed in newly emerging leaves of a rooted black currant cutting affected by severe reversion disease. From the symptom-bearing leaves, a virus was mechanically transmitted (with difficulty) to Chenopodium quinoa and was then transmitted from C. quinoa to other herbaceous test plants. The virus was purified and partially characterized, and the purified virions were used for antiserum production. Virus particles were isometric, c. 27 nm diam. and sedimented as 2 nucleoprotein components. They contained a protein species with a molecular mass of 55 kDa, which was readily degraded into a 54-kDa protein and 2 major RNA components of c. 6700 and 7700 nucleotides, each with a poly(A) tail. Most of these properties are shared by nepoviruses, but the virus was serologically unrelated to 14 nepoviruses or putative nepoviruses tested. The deduced sequence of 1260 nucleotides at the 3' end of 1 of the viral RNA species was distinct from any known viral sequence, except that it contained short regions of homology to the 3' terminal sequences of RNAs of 7 other nepoviruses and 2 comoviruses. To detect this virus in Ribes, primers were designed from the known sequence to amplify a 210-nucleotide region of the cDNA of the virus RNA using immunocapture reverse transcriptase-PCR (IC-RT-PCR). Using this assay, the virus was associated with 2 recognized forms of black currant reversion disease occurring in Finland, Scotland or New Zealand. The virus was detected in vector gall mites from reverted plants and in black currant plants on which such vector mites had fed. However, it was not detected by IC-RT-PCR in: known healthy Ribes plants; in Ribes plants free from reversion but affected by 3 other distinct virus-like diseases of Ribes; nor in plants infected with arabis mosaic, strawberry latent ringspot or raspberry ringspot nepoviruses. It is suggested that this virus is the causal agent of reversion disease, and the proposed name is black currant reversion associated virus.