Cytochrome c-553 of Desulfovibrio vulgaris, Miyazaki, was purified to homogeneity. The absorption spectrum of the ferro form has four peaks at 553, 525, 417 and 317 nm with a plateau near 280 nm, and that of the ferri form has three peaks at 525, 410 and 360 nm with a plateau near 280 nm and a shoulder at 560 nm. The millimolar absorbance coefficient of the alpha-peak of the ferro form is 23.9. The molecular weight of cytochrome c-553 is 8000, and it contains one heme. Its isoelectric point is rather alkaline, and its standard redox potential is -0.26 V at pH 7.0. Its amino acid composition is unique; it lacks proline, isoleucine and tryptophan. Ferrocytochrome c-553 does not combine with CO, nor does it transfer electrons directly to various redox carriers such as flavin nucleotides, methylene blue, indigodisulfonate, 5-methylphenazinium methyl sulfate, 1-methoxy-5-methylphenazinium methyl sulfate, viologens and cytochrome c3, but is oxidized by ferricyanide or by O2. Cytochrome c-553 can be reduced by formate dehydrogenase of this bacterium in the presence of formate, but not by hydrogenase under H2. The formate dehydrogenase does not reduce cytochrome c3 in the presence of formate. The systematic name for formate dehydrogenase of D. vulgaris is, therefore, established as formate:ferricytochrome c-553 oxidoreductase in EC subclass 1.22.-.