Crystallization of cytochrome c from horse, bovine, tuna and yeast (Iso-1) sources was attempted under similar conditions using ammonium sulfate and sodium nitrate as crystallization agents. With the exception of yeast cytochrome c, we were successful in obtaining crystals, however, in different forms. The requirement for similar amounts of the two salts suggested the importance of sodium nitrate for crystallization. That is, each salt plays a different role in the crystallization process: ammonium sulfate providing the ionic strength for the salting-out, sodium nitrate the specific intermolecular contacts. X-ray crystallographic analysis of the trigonal crystals of the horse protein revealed the existence of 2.5 nitrate ions per cytochrome c molecule in the crystal lattice. Morphodroms were made versus the concentrations of both salts. Their comparison clearly showed similarities and differences of the crystal habits among horse, bovine and tuna. The differences in the morphodroms correlated with the evolutionary divergence of the species. It was suggested that Asn103 played an important role in the crystal packing by interacting with one of the nitrate ions. This explains why, in the history of structure determination of this protein, only the crystals that were formed in the presence of two kinds of salts diffracted over 2.0 Angstrom resolution.