A gene cluster of 30 kilobases required for denitrification in Pseudomonas stutzeri ZoBell was identified and mapped. It harbors genes necessary for the respiratory reduction of nitrite (nir genes), nitric oxide (nor genes), and nitrous oxide (nos genes). Fifteen genes, 13 of which are transcribed in the same direction, have been located on a 56-kb BamHI fragment. They are arranged in three subclusters in the order nos-nir-nor.