Two kinds of C-type cytochromes, cytochrome c-552 and cytochrome c-554, derived from the chemoautotroph, Nitrosomonas europaea, were purified to an electrophoretically homogeneous state and their properties determined. Cytochrome c-552 possessed absorption peaks at 410 nm in the oxidized form, and at 416, 523, and 552 nm in the reduced form. The isoelectric point of the cytochrome was at pH 3.7, its midpoint redox potential was about +0.25 V and its molecular weight about 10,000. The cytochrome reacted rapidly with Pseudomonas aeruginosa nitrite reductase [EC 1.9.3.2] and at an appreciable rate with a cytochrome oxidase preparation derived from N.europaea, but did not react either with cow cytochrome oxidase [EC 1.9.3.1] or with yeast cytochrome c peroxidase [EC 1.11.1.5]. Cytochrome c-554 had absorption peaks at 407 nm in the oxidized form, and at 421, 524, and 554 nm in the reduced form. The isoelectric point of cytochrome c-554 was at pH 10.7. The molecular weight of this cytochrome was 10,700 on the basis of the heme content and dry weight, while it was found to be 21,500 by gel filtration with a Sephadex G-100 column and 25,000 by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulphate. Cytochrome c-554 was autoxidizable. Hydroxylamine-cytochrome c reductase (hydroxylamine: cytochrome c oxidoreductase, [EC 1.7.3.4]) of N.europaea did not react with cytochrome c-552, while it reduced the cytochrome with hydroxylamine in the presence of cytochrome c-554. On the basis of these facts, together with the reactivity of cytochrome c-552 with N. europaea cytochrome oxidase, the electron transfer mechanisms coupled to the oxidation of hydroxylamine in the organism were discussed. The evolutionary position of N. europaea was also discussed on the basis of the enzymatic properties of cytochrome c-552.