Four different membrane-bound b-type cytochromes were found to occur in Halobacterium halobium strain L-33, and two of them, b-558 and b-562, were purified to homogeneity. Cytochrome b-558 showed absorption peaks at 414 and 526 nm in the oxidized form, and peaks at 425, 528, and 558 urn in the reduced form. Its α peak at 558 nm in the reduced form was asymmetric with a shoulder at around 554 nm. At liquid nitrogen temperature, the α peak was split into two peaks at 549 and 556 nm which appeared to be the α peaks of cytochromes c and b, respectively. The cytochrome contained 1 mol of protoheme in 28, 500g, and was composed of one molecule each of two subunits with molecular masses of 15.4 and 11.7 kDa, respectively. The heme seemed bound to the larger subunit. The cytochrome was very autoxidizable and its redox potential at pH 8.0 was -75 mV. Cytochrome b-562 showed absorption peaks at 417 and 530 nm in the oxidized form and peaks at 431, 531, and 562 nm in the reduced form. The cytochrome was composed of only one polypeptide (25kDa) and seemed to contain one protoheme molecule per molecule.