From Nitrosomonas europaea which had been cultivated in a medium deficient in copper, cytochrome c oxidase (aa3-type) which did not have CuA was purified. The oxidase did not show the 830-nm peak and its ESR spectrum differed greatly from that of the normal enzyme, which has two copper atoms, CuA and CuB, per molecule. However, the oxidase which did not have CuA showed almost the same cytochrome c oxidizing activity as the normal oxidase.