The crystal structure of an unusual monomeric cytochrome c' from Rhodopseudomonas palustris (RPCP) has been determined at 2.3 Å resolution. RPCP has the four-helix (Helix A, B, C and D) bundle structure similar to dimeric cytochromes c( However the amino acid composition of the surface of Helix A and B in RPCP is remarkably different from that of the dimeric cytochromes c( This surface forms the dimer interface in the latter proteins. RPCP has seven charged residues on this surface contrary to the dimeric cytochromes c' which have only two to three charged groups on the corresponding surface. Moreover, hydrophobic residues on this surface of RPCP are two to three times fewer than in dimeric cytochromes ct We thus suggest that RPCP is monomeric in solution because of the hydrophilic nature of the A-B surface. Cytochrome c'can be classified into two types. fype I cytochromes c' have hydrophobic A-B surfaces and they are globular. The A-B surface of type 2 cylochromes c' is hydrophilic and they take a monomeric or flattened dimeric form.