The virulent Salmonella tl/phimurium strain WB600 carries the mviA allele of the gene mouse virulence A. As shown here, the virulent phenotype of WB600 is the result of a nonfunctional mviA gene. As compared to the functional allele mviA +, mviA increases virulence in Ity5 mice, but not in Ity r mice. A specific BgllI site, mviA4185, between osraZ and gaIU, located at ˜35 min on the salmonella chromosome, was within mviA. Insertion of an antibiotic cassette in the mviA4185 site of mviA or the homologous mviA4093 site of mviA DNA resulted in virulence when either cassette was recombined into the chromosome. When mviA and mviA+ were both expressed in the same strain with one carried in the chromosome and the other on a plasmid, avirulence was dominant. Replacement of the mviA allele of strain WB600 using P22 transductions of linked antibiotic cassettes cloned into the chromosome of virulent S. typhimurium strains (SR- 11, TML, SLD44, C5, ATCC14028, Wl18-2, and WB600) showed that all but WB600 contained the avirulent mviA allele. Southern hybridizations provided no evidence for a second mviA allele anywhere in the genome of the six non-WB600 strains. © 1991, Rockefeller University Press., All rights reserved.