Recently high levels of protection against tomato spotted wilt virus (TSWV), a negative-strand RNA virus infecting plants, have been obtained by transforming tobacco with viral nucleoprotein (N) gene sequences. Here we demonstrate that this protection is primarily due to the presence of N gene transcripts in the cells of transgenic plants, and hence appears to be RNA-mediated. Further, transgenic tobacco plants are only protected to isolates and strains of TSWV and not to other tospoviruses that share considerable nucleotide sequence homology in their N genes to TSWV. In addition to being protected after mechanical inoculation, the transgenic tobacco plants are also resistant to inoculation using viruliferous thrips, i.e. Frankliniella occidentalis (Perg.), one of the most important natural vector species.