Pathogenicity islands are chromosomal clusters of horizontally acquired virulence genes that are often found at tRNA loci. The selC tRNA locus of Escherichia coli has served as the site of integration of two distinct pathogenicity islands which are responsible for converting benign strains into uro- and enteropathogens. Because virulence genes are targeted to the selC locus of E.coli, we investigated the homologous region of the Salmonella typhimurium chromosome for the presence of horizontally acquired sequences. At this site, we identified a 17 kb DNA segment that is both unique to Salmonella and necessary for virulence. This segment harbors a gene, mgtC, that is required for intramacrophage survival and growth in low Mg2+ media. The mgtC locus is regulated by the PhoP/PhoQ two-component system, a major regulator of virulence functions present in both pathogenic and non-pathogenic bacterial species. Cumulatively, our experiments indicate that the ability to replicate in low Mg2+ environments is necessary for Salmonella virulence, and suggest that a similar mechanism is responsible for the dissemination and acquisition of pathogenicity islands in enteric bacteria.