Beef heart mitochondrial ubiquinol:cytochrome c oxidoreductase has been crystallized in the shape of hexagonal bipyramids. At present the crystals diffract X-rays to 4.7 A. From preliminary analysis the diffraction pattern appears to be consistent with space group P6(1)22 or P6(5)22 and with unit cell parameters a = b = 212 A and c = 352 A.