The transgenic sugar beet used in our experiments was made available by Kleinwanzlebener Saatzucht AG-PLANTA Corporation, Einbeck, Germany. Our transgenic genotype is a conventional sugar beet en- gineered with the coat protein gene of BNYVV (as reported in Mannerlof et al. 1996). Other transgenes in the genome are a herbicide resistance (bar) gene against phosphinothricin, and an antibiotic resistance (nptll) gene against kanamycin, which were used as selectable markers. The parent of the transgenic beets was an inbred sugar beet line (97% homozygosity). We used this line as a parent for our Fl control. The transgenic inbred line had 98% homozygosity as a result of selfing. Both the transgenic and nontransgenic sugar beet lines were hand-crossed to an emasculated nontransgenic Swiss chard cultivar ("Glatter Silber") to obtain Fl hybrids (= "transgenic hybrid," and "nontransgenic hybrid") (Bartsch and Pohl-Orf 1996). To verify successful out- crossing, we tested for bar gene expression in the pre- sumed transgenic Fl progeny by applying phosphin- othricin to a representative sample. Approximately 1% of the progeny succumbed to the herbicide, apparently the result of Swiss chard self-pollination, probably due to incomplete emasculation. In all experiments, the Swiss chard variety was also used as an additional con- trol. This genotype, like the control inbred sugar beet line, is not virus tolerant. The field tests were conducted in 1996 at a site in Mainz (Rheinland-Pfalz, Germany) with (1) low back- ground virus infection plots, intermixed with (2) high background virus infection (experimental plants inoc- ulated with the virus) plots. The plants for the high- infestation treatment were inoculated as young seedment