Abstract Cytochrome c 553 of Desulfovibrio , vulgaris differs in amino acid composition and in molecular weight (9, 100 ± 100) from cytochrome c 3 (12,000) of the same organism. This protein can be easily obtained in crystalline form. It consists of a single polypeptide chain comprising about 80 amino acid residues and bearing a single heme group. Both termini are freely available; alanine and leucine are marking the amino terminal and the carboxyl terminal position, respectively. In addition to cytochrome c 553 and to cytochrome c 3 a third c-type cytochrome, of larger molecular weight, was detected in D. , vulgaris and related organisms.