Tobacco ringspot virus (TobRV) is the type member of the nepoviruses1. It consists of 28-nm isometric particles which contain one or the other of the two single-strand genomic RNAs of 4.8 and 7.2 kilobases (kb) (refs 2 and 3). TobRV infects a wide range of dicotyledonous plants and is the causative agent of the budblight disease of soybean. A small RNA which can replicate to high levels and be encapsidated by TobRV in infected plants has been found during serial passages of virus isolates4,5. It is not required for virus propagation and has no detectable sequence homology with the virus genomic RNAs. It is therefore termed the satellite RNA of tobacco ringspot virus (STobRV). It can be considered a parasite of the virus and it ameliorates disease symptoms when present during virus infection of plants. We report here the expression of forms of the STobRV sequence in transgenic tobacco plants. Plants which express full-length STobRV or its complementary sequence as RNA transcripts show phenotypic resistance when infected with TobRV. This is correlated with the amplification of satellite RNA to high levels during virus infection of plants.