The terminal oxidoreductase of nitrous oxide respiration in the marine, denitrifying bacterium, Pseudomonas perfectomarinus, was identified as multi-copper protein and purified to electrophoretic homogeneity. The enzyme reduced N20 to N2 with hydrogen, clostridial hydrogenase, and methyl viologen as electron-donating system. The copper content of the reductase corresponded to ~8 copper atoms/120000 Mr. The subunit structure was dimeric with two peptides of equal size. Manganese, iron and zinc were absent, or were not found in stoichiometric amounts. The oxidized chromophore had absorption maxima at 350, 480, 530, 620 and 780 nm; addition of dithionite produced a blue protein form with maxima at 470, 635 and 740 nm. Both forms of the enzyme were paramagnetic. The same copper protein was also isolated from Pseudomonas stutzeri.