CcmG/DsbE is a typical thiol/disulfide oxidoreductase, exhibiting a specific reducing activity in a highly oxidizing environment, and is involved in electron transfer during the maturation of c-type cytochromes. Escherichia coli CcmG/DsbE (residues 19-185) has been crystallized using the hanging-drop vapour-diffusion technique. The crystals belong to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 35.48, b = 48.52, c = 84.78 A. X-ray data have been collected to 1.9 A resolution.