SUMMARY: The soluble cytochrome oxidase of Nitrosomonas europaea has been highly purified and shown to be a copper protein devoid of haem, not a cytochrome o as was previously assumed. The native molecular weight was 120000 and the subunit molecular weight 35000. Soluble cytochrome oxidase activity co-purified with nitrite reductase activity; both activities were almost certainly associated with the same protein. The possible physiological role of the nitrite reductase activity is discussed.