The purification, crystallization and X-ray diffraction analysis of Saccharomyces cerevisiae Delta(3)-Delta(2)-enoyl-CoA isomerase is described. Delta(3)-Delta(2)-Enoyl-CoA isomerase is a member of the hydratase/isomerase protein family and is an auxiliary enzyme required for the beta-oxidation of unsaturated fatty acids. It is a hexameric enzyme consisting of six identical 32 kDa subunits of 280 residues each. In crystallization trials three crystal forms were obtained, with tetragonal and hexagonal lattices. A 2.5 A data set was collected from the unliganded hexagonal crystals with an R(merge) of 6.6%. The crystal, with unit-cell parameters a = 116.0, b = 116.0, c = 122.9 A, is likely to have P6(3)22 symmetry.