The bifunctional flavoenzyme 5-hydroxyvaleryl-CoA dehydratase/dehydrogenase has been crystallized from solutions containing ammonium sulfate (form I) or polyethylene glycol (form II) as precipitant. In both cases, the crystals grew in the monoclinic space group C2. The unit cell dimensions for form I crystals were determined as a = 162.8 A, b = 71.8 A, c = 83.5 A, beta = 109.1 degrees; corresponding values for form II crystals were a = 161.2 A, b = 71.6 A, c = 82.2 A, beta = 109.3 degrees. In both cases most probably there are two monomers per asymmetric unit. The crystals diffract to about 2 A resolution and are rather stable in the X-ray beam.