A cDNA clone of cucumber mosaic virus (CMV) 117 N-satellite RNA driven by the cauliflower mosaic virus (CaMV) 35S transcript promoter, was stably integrated into the genome of Petunia hybrida `Bluepicoti' tissues by Agrobacterium tumefaciens Ti plasmid-mediated transformation. Transgenic plants producing CMV satellite RNA showed delayed disease development when inoculated with CMV-Y, a helper virus for the I17N-satellite RNA. Furthermore, transgenic petunia plants showed delayed disease development against tobacco mosaic virus (TMV), a tobamovirus not related to CMV. Northern blot analysis revealed that large amounts of unit length satellite RNA (335 bp) were produced in CMV-infected transgenic petunia plants; whereas, mainly transcripts driven by the CaMV 35S promoter (approximately 1 kb) were produced in TMV-infected transgenic plants. SDS-PAGE and Western blotting showed that symptom reduction was correlated with a reduction in the amount of viral coat protein in transgenic plants.