Regulation by oxygen of the peptidase T (pepT) locus of Salmonella typhimurium was studied by measuring beta-galactosidase levels in strains containing a pepT::Mu d1(Apr lac) operon fusion. beta-Galactosidase was induced in anaerobic cultures and late-exponential and stationary-phase aerated cultures. Peptidase T activity also was induced under these growth conditions. pepT+ but not pepT strains will utilize as amino acid sources the tripeptides Leu-Leu-Leu and Leu-Gly-Gly only when grown anaerobically. Mutations at two loci, oxrA and oxrB (oxygen regulation) prevent induction of the pepT locus. The oxrA locus is homologous to the fnr locus of Escherichia coli. We have isolated 12 independent Mu d1 insertions (oxd::Mu d1, oxygen dependent) that show induction of beta-galactosidase in anaerobic cultures and stationary-phase aerated cultures. These insertions fall into nine classes based on map location. All of the oxd::Mu d1 insertions are regulated by oxrA and oxrB and therefore define a global regulon that responds to oxygen limitation.