A mutant of Pseudomonas aeruginosa was characterized which could not grow anaerobically with nitrate as the terminal electron acceptor or with arginine as the sole energy source. In this anr mutant, nitrate reductase and arginine deiminase were not induced by oxygen limitation. The anr mutation was mapped in the 60-min region of the P. aeruginosa chromosome. A 1.3-kb chromosomal fragment from P. aeruginosa complemented the anr mutation and also restored anaerobic growth of an Escherichia coli fnr deletion mutant on nitrate medium, indicating that the 1.3-kb fragment specifies an FNR-like regulatory protein. The arcDABC operon, which encodes the arginine deiminase pathway enzymes of P. aeruginosa, was rendered virtually noninducible by a deletion or an insertion in the -40 region of the arc promoter. This -40 sequence (TTGAC....ATCAG) strongly resembled the consensus FNR-binding site (TTGAT....ATCAA) of E. coli. The cloned arc operon was expressed at low levels in E. coli; nevertheless, some FNR-dependent anaerobic induction could be observed. An FNR-dependent E. coli promoter containing the consensus FNR-binding site was expressed well in P. aeruginosa and was regulated by oxygen limitation. These findings suggest that P. aeruginosa and E. coli have similar mechanisms of anaerobic control.