Pyridoxine 5'-phosphate oxidase (PNP Ox) catalyzes the terminal step in the biosynthesis of pyridoxal 5'-phosphate. The 53-kDa homodimeric enzyme contains a noncovalently bound flavin mononucleotide (FMN) on each monomer. Three crystal forms of Escherichia coli PNP Ox complexed with FMN have been obtained at room temperature. The first crystal form belongs to trigonal space group P3(1)21 or P3(2)21 with unit cell dimensions a = b = 64.67 Angstrom, c = 125.64 Angstrom and has one molecule of the complex (PNP Ox-FMN) per asymmetric unit. These crystals grow very slowly to their maximum size in about 2 to 4 months and diffract to about 2.3 Angstrom. The second crystal form belongs to tetragonal space group P4(1) or P4(3) with unit cell dimensions a = b = 54.92 Angstrom, c = 167.65 Angstrom, and has two molecules of the complex per asymmetric unit. The crystals reach their maximum size in about 5 weeks and diffract to 2.8 Angstrom. A third crystal form with a rod-like morphology grows faster and slightly larger than the other two forms, but diffracts poorly and could not be characterized by X-ray analysis. The search for heavy-atom derivatives for the first two crystal forms to solve the structure is in progress.