Cytochrome c551 peroxidase is a periplasmic enzyme expressed in Pseudomonas aeruginosa at low oxygen tensions. The glycosylated enzyme has been purified to homogeneity and crystallized by vapour diffusion techniques using polyethylene glycol 2000 as the precipitant in the presence of isopropanol. The crystals belong to the trigonal space group P3(1)21 or P3(2)21 with unit cell dimensions of a = b = 113.8 A, c = 72.0 A. They are suitable for X-ray analysis and diffract to dmin = 2.5 A. There is one peroxidase molecule in the crystallographic asymmetric unit.