Abstract An acidic cytochrome c (P i = 4.8) has been purified from Desulfovibrio desulfuricans Norway. Its molecular weight was estimated to be 26,000 but a monomeric form of 13,500 molecular weight has been obtained. The comparison of its amino acid composition and N terminal sequence has characterized this cytochrome as a new cytochrome, different from cytochrome c 3 (Mr 13,000) and cytochrome c 553(550) studied in the same organism. Its optical spectrum was similar to cytochrome c 3 (Mr 13,000) accordingly it has 4 haems per subunit. The absence of absorption at 695 nm indicates that two histidine residues are implicated as fifth and sixth ligand for haem iron. This new cytochrome is homologous to the cytochrome C 3 (Mr 26,000) previously described for Desulfovibrio gigas and Desulfovibrio vulgaris .