Recombinant mature xylanase B from Clostridium stercorarium has been prepared and crystallized by the sitting-drop vapour-diffusion method using 4 mg ml(-1) purified enzyme, 10.3%(w/v) polyethylene glycol 1500, 8.6%(v/v) glycerol and 0.34 M non-detergent sulfobetaine 195. A suitable crystal grew after incubation for ten weeks at 293 K. The crystal belonged to space group P2(1)2(1)2(1), with unit-cell parameters a = 64.76, b = 96.60, c = 138.44 A. X-ray diffraction data were collected to 1.80 A resolution.