The molecular structure of the cytochrome c2, isolated from the purple photosynthetic bacterium Rhodobacter capsulatus, has been solved to a nominal resolution of 2.5 A and refined to a crystallographic R-factor of 16.8% for all observed X-ray data. Crystals used for this investigation belong to the space group R32 with two molecules in the asymmetric unit and unit cell dimensions of a = b = 100.03 A, c = 162.10 A as expressed in the hexagonal setting. An interpretable electron density map calculated at 2.5 A resolution was obtained by the combination of multiple isomorphous replacement with four heavy atom derivatives, molecular averaging and solvent flattening. At this stage of the structural analysis the electron densities corresponding to the side-chains are well ordered except for several surface lysine, glutamate and aspartate residues. Like other c-type cytochromes, the secondary structure of the protein consists of five alpha-helices forming a basket around the heme prosthetic group with one heme edge exposed to the solvent. The overall alpha-carbon trace of the molecule is very similar to that observed for the bacterial cytochrome c2, isolated from Rhodospirillum rubrum, with the exception of a loop, delineated by amino acid residues 21 to 32, that forms a two stranded beta-sheet-like motif in the Rb. capsulatus protein. As observed in the eukaryotic cytochrome c proteins, but not in the cytochrome c2 from Rsp. rubrum, there are two evolutionarily conserved solvent molecules buried within the heme binding pocket.