The characterization of the hydroxylamine oxidase from the heterotrophic nitrifier Paracoccus denitrificans GB17 indicates the enzyme to be entirety distinct from the hydroxylamine oxidase from the autotrophic nitrifier Nitrosomonas europaea. Hydroxylamine oxidase from P. denitrificans contains three to five non-haem, non-iron-sulphur iron atoms as prosthetic groups, predominantly co-ordinated by carboxylate ligands. The interaction of the enzyme with the electron-accepting proteins cytochrome c550 and pseudoazurin is mainly hydrophobic. The catalytic mechanism of hydroxylamine oxidase from P. denitrificans is different from the enzyme from N. europaea because the production of nitrite by the former requires molecular oxygen. Under anaerobic conditions the enzyme makes nitrous oxide as a sole product.