Two kinds of C-type cytochromes, cytochrome c-550 and cytochrome c-551, derived from the facultative chemoautotroph, Thiobacillus novellus, were highly purified. Cytochrome c-550 possessed absorption peaks at 410 mμ in the oxidized form, and at 414.5, 520 and 550 mμ in the reduced form. The isoelectric point of the cytochrome was at pH7.5, and the midpoint redox potential+0.276V. Amino acid analysis showed that cytochrome c-550 was composed of 118 amino acid residues and its molecular weight was 13, 270. The cytochrome reacted fairly rapidly with cow cytochrome oxidase [EC 1.9.3.1], very rapidly with yeast cytochrome c peroxidase [EC 1.11.1.5], and poorly with Pseudomonas aeruginosa nitrite reductase [EC 1.9.3. 2]. Cytochrome c-551 had absorption maxima at 410.5 mμ in the oxidized form, and at 416, 522 and 551 mμ in the reduced form. The isoelectric point and midpoint redox potential of cytochrome c-551 were at pH5.2 and +0.260V, respectively. The cytochrome did not react with either of the animal cytochrome oxidase, the yeast peroxidase and the bacterial nitrite reductase. Sulphite: cytochrome c oxidoreductase partially purified from the organism reduced very rapidly both the C-type cytochromes with sulphite as the electron donor. The enzyme reacted rapidly also with cow and yeast cytochromes c, but did not react with cytochrome c (551, Pseudomonas aeruginosa), cytochrome c (552, Pseudomonas stutzeri) or cytochrome c (553, Endarachne binghamiae). The evolutionary position of T. novellus was discussed on the basis of the enzymatic properties of cytochrome c-550 and of the specificity of sulphite: cytochrome c oxidordectase for cytochrome c used as the electron acceptor.