The major flavin reductase from Vibrio fischeri FRase I, has been crystallized in the presence of FMN by the vapor diffusion method using polyethylene glycol 4000 as a precipitant. The crystals belonged to the monoclinic space group C2 with unit cell dimensions, a = 101.6 Angstrom, b = 63.2 Angstrom, c = 74.4 Angstrom and beta = 100.0 degrees. The crystals are expected to contain two FRase I molecules per asymmetric unit. The crystals diffracted X-rays to at least 2.2 Angstrom resolution and are appropriate for structural analysis at high resolution. (C) 1996 Academic Press, Inc.