Crystals of a bacterial glutathione S-transferase from Proteus mirabilis have been grown from polyethylene glycol by the hanging-drop vapour-diffusion method. Successful crystallization required the presence of the substrate glutathione. The crystals belong to the tetragonal space group P4 with cell dimensions a = b = 90.9 and c = 117.3 A. They contain between three and six monomers in the asymmetric unit and diffract to beyond 2.3 A resolution.