The [NiFeSe] hydrogenases belong to a subgroup of the [NiFe] proteins in which a selenocysteine is a ligand of the Ni. These enzymes demonstrate interesting catalytic properties, showing a very high H(2)-producing activity that is sustained in the presence of low O(2) concentrations. The purification, crystallization and preliminary X-ray diffraction analysis of the [NiFeSe] hydrogenase isolated from Desulfovibrio vulgaris Hildenborough are reported. Crystals of the soluble form of this hydrogenase were obtained using 20% PEG 1500 as a precipitant and belonged to the monoclinic space group P2(1), with unit-cell parameters a = 60.57, b = 91.05, c = 66.85 A, beta = 101.46 degrees. Using an in-house X-ray diffraction system, they were observed to diffract X-rays to 2.4 A resolution.