Nitric oxide is a key element in host defense against invasive pathogens. The periplasmic cytochrome cnitrite reductase (NrfA) of Escherichia coli catalyzes the respiratory reduction of nitrite, but in vitro studies have shown that it can also reduce nitric oxide. The physiological significance of the latter reaction in vivo has never been assessed. In this study the reduction of nitric oxide byEscherichia coli was measured in strains active or deficient in periplasmic nitrite reduction. Nrf+ cells, harvested from cultures grown anaerobically, possessed a nitric-oxide reductase activity with physiological electron donation of 60 nmol min−1· mg dry wt−1, and an in vivo turnover number of NrfA of 390 NO⋅ s−1was calculated. Nitric-oxide reductase activity could not be detected in Nrf− strains. Comparison of the anaerobic growth of Nrf+ and Nrf− strains revealed a higher sensitivity to nitric oxide in the NrfA− strains. A higher sensitivity to the nitrosating agentS-nitroso-N-acetyl penicillamine (SNAP) was also observed in agar plate disk-diffusion assays. Oxygen respiration by E. coli was also more sensitive to nitric oxide in the Nrf− strains compared with the Nrf+ parent strain. The results demonstrate that active periplasmic cytochromec nitrite reductase can confer the capacity for nitric oxide reduction and detoxification on E. coli. Genomic analysis of many pathogenic enteric bacteria reveals the presence ofnrf genes. The present study raises the possibility that this reflects an important role for the cytochrome cnitrite reductase in nitric oxide management in oxygen-limited environments.