A gene whose transcript bears a monomeric form of cucumber mosaic virus (CMV) satellite RNA was introduced into tobacco (Nicotiana tabacum 'Xanthi' nc) plants by using an Ri plasmid-based vector system. On CMV infection, the transcript of the satellite RNA gene was used as a template to yield unit-length satellite RNA, which was efficiently amplified by the virus. Plants bearing the satellite RNA gene displayed long-term tolerance to CMV infection and were also tolerant to CMV infection by aphids, the natural vector of CMV. Implications of these results concerning the mechanism of satellite RNA replication are discussed.