Rat liver arginase, a manganese-metalloenzyme, has been crystallized from polyethylene glycol 8000 in N,N-bis(2-hydroxyethyl)glycine (Bicine) buffer at pH 8.5. Crystals form as either cubes or pyramids and belong to space group P31 (or P32) with hexagonal unit cell dimensions a = b = 88·9 Å, c = 114·8 Å, or a = b = 88·5 Å, c = 104·5 Å; the variation along the c axis does not correlate with the external crystal morphology of cube or pyramidshaped. X-ray diffraction data are measured to a limiting resolution of 2·4 Å…. Given the volume constraints of the unit cell it is likely that rat liver arginase is a trimer, with three 35,000 Da monomers in the asymmetric unit. This resolves a persistent ambiguity regarding the oligomeric structure of this enzyme.