The lambdoid phage Gifsy-2 contributes significantly to Salmonella enterica serovar Typhimurium virulence. The phage carries the periplasmic superoxide dismutase gene, sodCI, and other unidentified virulence factors. We have characterized the gene grvA, a single open reading frame inserted in the opposite orientation in the tail operon of the Gifsy-2 phage. Contrary to what is observed with classic virulence genes, grvA null mutants were more virulent than wild type as measured by intraperitoneal competition assays in mice. We have termed this effect antivirulence. Wild-type grvA in single copy complemented this phenotype. However, grvA(+) on a multicopy plasmid also conferred the antivirulence phenotype. Neither a grvA null mutation nor the grvA(+) plasmid conferred a growth advantage or disadvantage in laboratory media. The antivirulence phenotype conferred by the grvA null mutation and the grvA(+) plasmid required wild-type sodCI but was independent of other virulence factors encoded on Gifsy-2. These results suggest that in a wild-type situation, GrvA decreases the pathogenicity of serovar Typhimurium in the host, most likely by affecting resistance to toxic oxygen species. These virulence phenotypes were independent of functional Gifsy-2 phage production. Our data suggest that the contribution of Gifsy-2 is a complicated sum of both positive virulence factors such as sodCI and antivirulence factors such as grvA.