The water-soluble portion of rat heme oxygenase-1 which lacks 22 hydrophobic amino-acid residues at the C-terminus was expressed in E. coli and crystallized in the form of a complex with heme by the vapor-diffusion method using polyethylene glycol 4000 as the precipitant. The crystals belong to the tetragonal space group P41212 or P43212, with unit-cell dimensions of a = b = 56.7, c = 186.7 Å. The crystal contains one heme–heme oxygenase-1 complex in an asymmetric unit and diffracts X-rays beyond 3.0 Å resolution with a conventional X-ray source.