By using MudJ (Kan, lac)-directed operon fusion technology, mutants of Salmonella typhi whose gene expression is induced under anaerobic growth conditions were isolated. Characterization of their phenotypes and regulatory properties revealed that two of the mutants were unable to use nitrate as a terminal electron acceptor in the absence of oxygen, suggesting that they were defective in nitrate reductase activity. Anaerobic induction of these fusions did not further increase in response to nitrate. Strains carrying an additional mutation in oxrA were constructed. They showed a lower level of beta-galactosidase expression both aerobically and anaerobically; however, the ratios of anaerobic induction remained unaltered. These MudJ insertions mapped to the 17-19 min region of the chromosome. Based upon their phenotypes and mapping, one of the mutants probably possessed a modC (chlD)::MudJ insertion and the other a moaA (chlA)::MudJ insertion. A third mutant was unable to use either nitrate or fumarate as a terminal electron acceptor. All three mutants showed a reduced ability to enter into and proliferate within HEp-2 epithelial cells. The oxrA mutation enhanced entry and proliferation of both the wild-type cells and the three mutants. Taken together, these results suggest that anaerobic respiration plays a role in S. typhi invasiveness.