The PhoP-PhoQ two-component system is essential for virulence in Salmonella typhimurium. This system controls expression of some 40 different proteins, yet most PhoP-regulated genes remain unknown. To identify PhoP-regulated genes, we isolated a library of 50,000 independent lac gene transcriptional fusion strains and investigated whether production of beta-galactosidase was regulated by PhoP. We recovered 47 lac gene fusions that were activated and 7 that were repressed when PhoP was expressed. Analysis of 40 such fusions defined some 30 loci, including mgtA and mgtCB, which encode two of the three Mg2+ uptake systems of S. typhimurium; ugd, encoding UDP-glucose dehydrogenase; phoP, indicative that the phoPQ operon is autoregulated; and an open reading frame encoding a protein with sequence similarity to VanX, a dipeptidase required for resistance to vancomycin. Transcription of PhoP-activated genes was regulated by the levels of Mg2+ in a PhoP-dependent manner. Strains with mutations in phoP or phoQ were defective for growth in low-Mg2+ media. The mgtA and mgtCB mutants reached lower optical densities than the wild-type strain in low-Mg2+ liquid media but displayed normal growth on low-Mg2+ solid media. Six PhoP-activated genes were identified as essential to form colonies on low-Mg'+ solid media. Cumulatively, our experiments establish that the PhoP-PhoQ system governs the adaptation to magnesium-limiting environments.