Human argininosuccinic acid lyase (ASAL) has been expressed, purified and crystallized in several distinct crystal morphologies. At present only one form is suitable for X-ray diffraction analysis. These crystals grow as hexagonal prisms, with unit cell dimensions a = b = 104.6 A, c = 185.3 A and alpha = beta = 90 degrees, gamma = 120 degrees. The crystals exhibit the symmetry of space group P3(1)21 or its enantiomorph, P3(2)21 (indistinguishable crystallographically) and diffract to a minimum d-spacing of approximately 3.5 A.