The particles of cultures of cucumber mosaic virus (CMV) contain the three genomic species of single-stranded RNA and a sub-genomic species that acts as messenger RNA for CMV particle protein. Some cultures also contain a single-stranded linear RNA molecule that is typically ~335 nucleotides long1,2. This extra molecule, termed satellite RNA, does not share appreciable nucleotide sequence with CMV genomic RNA3 but replicates only in plants1 or protoplasts4 that are infected with CMV. CMV isolates that do not contain satellite RNA can be cultured repeatedly in plants in a satellite-free state, but when satellite RNA is added to such cultures it is synthesized and persists as a component of the virus isolate5. The effect of satellite RNA on CMV infections depends on the strain of satellite: in many cases the usual symptoms of CMV are suppressed and as a result the infected plants show few symptoms of infection6-8. However, the presence of other strains of satellite RNA leads to the production of severe symptoms that are quite distinct from those of CMV8. Here we describe the transformation of tobacco plants with DNA copies of CMV satellite RNA, the production of satellite RNA transcripts by such plants and the acquisition of satellite RNA by CMV cultures grown in them. These results suggest a means of protecting plants against the effects of CMV and also suggest a method by which satellite RNAs may have evolved.