Virus-resistant transgenic plants have been created primarily through the expression of viral sequences.  It has been hypothesized that recombination between the viral transgene mRNA and the RNA of an infecting virus could generate novel viruses.  As mRNA/viral RNA recombination can occur in virus-resistant transgenic plants, the key to testing this risk hypothesis is to compare the populations of recombinant viruses generated in transgenic and non-transgenic plants.  This has been done with two cucumoviral systems, involving either two strains of cucumber mosaic virus (CMV), or CMV and the related tomato aspermy virus (TAV).  Although the distribution of the sites of recombination in the CMV/CMV and TAV/CMV systems was quite different, equivalent populations of recombinant viruses were observed in both cases.  These results constitute the first comparison of the populations of recombinants in transgenic and non-transgenic plants, and suggest that there is little risk of emergence of recombinant viruses in these plants, other than those that could emerge in non-transgenic plants.