Nitric oxide (NO) is present in soil and air, and is produced by bacteria, animals and plants. Superoxide (O^-₂) arises in all organisms inhabiting aerobic environments. Thus, many organisms are likely to encounter peroxynitrite (OONO^-), a product of NO and O^-₂ that forms at near diffusion-limited rates, and rapidly decomposes upon protonation through isomerization to nitrate (NO^-₃; ref. ) while generating hydroxyl radical (^.OH) and nitrogen dioxide radical (^.NO₂) (refs , ), both more reactive than peroxynitrite's precursors. The oxidative, inflammatory, mutagenic and cytotoxic potential (ref. ) of peroxynitrite contrasts with the anti-oxidant, anti-inflammatory and tissue-protective properties ascribed to NO itself. Thus, the ability of cells to cope with peroxynitrite is central in determining the biological consequences of NO production. We considered whether cells might be equipped with enzymes to detoxify peroxynitrite. Peroxiredoxins have been identified in most genomes sequenced, but their functions are only partly understood. Here we show that the peroxiredoxin alkylhydroperoxide reductase subunit C (AhpC) from Salmonella typhimurium catalytically detoxifies peroxynitrite to nitrite fast enough to forestall the oxidation of bystander molecules such as DNA. Results are similar with peroxiredoxins from Mycobacterium tuberculosis and Helicobacter pylori. Thus, peroxynitrite reductase activity may be widespread among bacterial genera.