The pJP4-encoded chloromuconate cycloisomerase, an enzyme of the 2,4-dichlorophenoxy-acetate degradation pathway, was purified from cell-free extracts of Alcaligenes eutrophus JMP134 with a revised procedure. Tetragonal bipyramidal crystals were grown and characterized with respect to their X-ray diffraction properties. They were assigned to the space group I4, with cell dimensions of a = b = 111.9 A, c = 148.5 A. The crystals scattered to approximately 3 A resolution.