Cytochrome c3 (M(r) 26000) isolated from Desulfovibrio gigas is a dimeric cytochrome consisting of two identical subunits of 109 amino acids, each of which contains four haem groups. On the basis of its amino acid sequence, this cytochrome clearly belongs to the cytochrome c3 superfamily, and will be classified in class III of the c-type cytochromes as defined by Ambler [(1980) in From Cyclotrons to Cytochromes (Robinson, A. B. and Kaplan, N. O., eds.), pp. 263-279, Academic Press, London]. It contains ten cysteine and nine histidine residues in each subunit, and eight cysteines and eight histidines linked to the four haem groups were found to be invariant on alignment of all known cytochrome c3 sequences. Two intermolecular disulphide bridges have been determined between cysteine residues 5 and 46 of the two monomers. Cytochrome c3 (M(r) 26,000) from D gigas is clearly different from cytochrome c3 (M(r) 13,000) from the same strain, with which it shows only 27% sequence identity. Compared with cytochrome c3 (M(r) 26,000) from D. desulfuricans Norway, the three-dimensional structure of which has been determined, 26.95% of the residues have been conserved. In the enzyme from D. desulfuricans Norway, hydrophobic interactions have been described across the dimer interface. Residues involved in similar interactions seem to be well conserved in the equivalent D. gigas cytochrome. This sequence provides structural data to allow specification of this new subclass of polyhaem cytochromes. Furthermore, D. gigas cytochrome c3 (M(r) 26,000) is the first polyhaem cytochrome shown to contain two disulphide bridges linking two identical subunits, which could induce more rigid folding. The folding and the evolution of this family of polyhaem cytochromes are discussed.