Cytochrome b(562) from Erwinia chrysanthemi has been crystallized using the hanging-drop vapour-diffusion method with ammonium sulfate as the precipitant. X-ray precession photographs show that the crystals formed belong to either of the enantiomorphic space groups P4(1)2(1)2 or P4(3)2(1)2 with the cell parameters a = b = 98.6 and c = 62.7 A. Estimation of the crystal density and consideration of the possible values for V(m) indicate that there is either a dimer or trimer in the asymmetric unit. Experiments using the synchrotron radiation source at the CCLRC Daresbury Laboratory have shown that the crystals diffract to at least 2.7 A resolution. An analysis of the N-terminal sequence indicates that this cytochrome shows limited homology to the cytochrome b(562) from E. coli. Determination of the structure will therefore allow analysis of the relationship between these two proteins.