Abstract From Nitrosomonas europaea, a cytochrome c′-type haem protein has been purified which has cytochrome- c peroxidase (ferrocytochrome- c:hydrogen-peroxide oxidoreductase, EC 188.8.131.52) activity. The protein has haem c and shows the high-spin-type absorption spectra: it shows absorption peaks at 400, 500 and 640 nm in the oxidized form and peaks at 433 and 553 nm in the reduced form. Its spectra are changed into the haemochrome-type at pH higher than 11. The absorption spectra of the CO and NO complexes of its reduced form and the NO complex of its oxidized form resemble the respective spectra of cytochromes c′ from other bacteria. However, the haem protein has some characteristics which distinguish it from other cytochromes c′; (1) the protein has a cytochrome- c peroxidase activity, (2) its Soret band is composed of a single peak, (3) its oxidized form reacts with cyanide and azide easily, and (4) its molecule is composed of a haem-containing protein subunit (15 kDa) and a non-haem protein subunit (10 kDa). The molecular activities of the protein as cytochrome- c peroxidase are 1010 and 243 per min with horse and N. europaea cytochromes c, respectively.