A bacterial thermophilic alcohol dehydrogenase which is stable and active at 85 degrees C, has been crystallized by vapor diffusion from solutions of polyethylene glycol. A monoclinic crystal form diffracts to 2.8 A resolution and belongs to space group C2 with unit cell dimensions a = 139.0 A, b = 137.4 A, c = 80.9 A and beta = 93.23 degrees. The asymmetric unit contains four molecules which exhibit 222 point symmetry. A second crystal form is orthohombic, space group P2(1)2(1)2 with unit cell dimensions a = 168.0 A, b = 123.0 A, c = 80.0 A, and it diffracts to 3.2 A resolution.