Satellite RNAs (satRNAs) depend on their helper viruses for replication, encapsidation and spread. The goal of the research was to determine helper virus specificity and host range of the RPV barley yellow dwarf luteovirus (BYDV-RPV) satRNA (satRPV RNA) and assay satRPV RNA for the ability to affect the accumulation and modulate disease symptoms of its helper viruses in protoplasts and plants. Although similar in structural genes, subgroup I and II luteoviruses have very different polymerases. BYDV-RPV and beet western yellows virus (BWYV), members of subgroup II luteoviruses, supported satRPV RNA replication in monocotyledonous and dicotyledonous hosts, respectively. In contrast, BYDV-PAV and ST9 associated RNA (ST9a RNA), subgroup I luteoviruses, failed to replicate satRPV RNA. However, the stimulation of BWYV by ST9a RNA resulted in an increased accumulation of satRPV RNA progeny in BWYV + ST9a RNA + satRPV RNA inoculated tobacco protoplasts. BWYV encapsidated satRPV RNA, but in a form different from that found in BYDV (RPV + PAV) particles. SatRPV RNA was transmitted to plants by aphids that acquired virus from infected protoplasts. The concentration of encapsidated helper and satRPV RNA had to be above a threshold level to facilitate aphid transmission. Oat plants infected with BYDV-RPV and satRPV RNA had milder symptoms than those infected with BYDV-RPV alone. SatRPV RNA reduced BYDV-RPV helper RNA accumulation in oat plants and protoplasts. However, it did not affect symptoms caused by the severe mixed infection of RPV and PAV BYDVs and had no effect on PAV RNA accumulation in oats. In contrast, satRPV RNA reduced the accumulation of both BWYV helper RNA and nonhelper ST9a RNA in shepherd's purse plants but attenuated BWYV and ST9a RNA symptoms only slightly. Sat RPV RNA symptom modulation seems to be determined by competition between the satRPV RNA and its helper virus for replication and encapsidation. The results showed that satRPV RNA can replicate in both monocotyledonous and dicotyledonous protoplasts and plants and suggested that the specificity determinants of satRPV RNA replication are contained within the polymerase genes of supporting viruses rather than in structural genes or host plants.