Tobacco bushy top disease is caused by a complex of the viruses tobacco bushy top virus (TBTV, a member of the genus Umbravirus) and tobacco vein distorting virus (TVDV, a member of the genus Polerovirus), which acts as a helper virus encapsidating the TBTV genomic RNA. RNA from purified virions is separated as five bands. The two largest (6.0 and 4.2 kb) were shown by Northern blot analysis to be the genomic RNAs of TVDV and TBTV, respectively. A band of about 3 kb was cloned and sequenced and shown to be the RNA of a previously undescribed virus with two open reading frames (ORFs), the second of which is an RNA-dependent RNA polymerase (RdRp) and is probably expressed by readthrough of the ORF1a stop codon. BLAST and phylogenetic analyses of the RdRp show that it is related to two RNAs previously reported in association with the poleroviruses Beet western yellows virus and Carrot red leaf virus. These three RNAs appear to represent species of a new genus of plant viruses dependent upon a helper polerovirus for their transmission.