Single crystals of cytochrome cd1 nitrite reductase from Thiosphaera pantotropha have been grown in the presence of ammonium sulphate by vapour diffusion techniques. Crystals are suitable for X-ray analysis and diffract to dmin = 1.5 A. They belong to space group P2(1) with unit cell dimensions of a = 107.8 A, b = 61.9 A, c = 101.2 A, beta = 112.5 degrees. The crystallographic asymmetric unit contains a dimer.