One important issue of biosafety research is whether gene flow from transgenic crops to nontransgenic relatives causes unwanted effects. We carried out field trials with hybrids between transgenic sugar beets, and a close cultivated relative, Swiss chard. This hybrid also acts as a model for 'weed beet' hybrids between sugar beet and wild/weed beet (Beta vulgaris ssp. maritima). Transgenic beets with beet necrotic yellow vein virus (BNYVV) coat protein (cp), phosphinothricin-acetyl-transferase (bar), and neomycin-phospho-transferase (nptII) genes were hand-crossed to Swiss chard. The resulting F1 plants and controls were grown at two different BNYVV infestation levels and three different competitive conditions with Chenopodium album. Transgenic hybrids had consistently higher biomass than controls under high background BNYVV infestation, and consistently lower biomass than controls under low background infestation. The transgenic hybrids had a significantly lower rate of bolting than controls at all sites. Competition with Ch. album always had a strong negative influence on the performance of all genotypes. We conclude that ecological implications due to the introduction and spread of virus-resistant transgenic hybrids will be observed only in those feral Swiss chard and wild beet populations where fitness is significantly influenced by high infestations of BNYVV.