We have obtained well-ordered single crystals of the flavoenzyme trypanothione reductase from Crithidia fasciculata. The crystals are tetragonal rods with unit cell dimensions a = 128.6 A, c = 92.5 A. The diffraction pattern corresponds to a primitive lattice. Laue class 4/m. Diffraction to better than 2.4 A has been recorded at the Daresbury Synchrotron. The accurate elucidation of the three-dimensional structure of this enzyme is required to support the rational design of compounds active against a variety of tropical diseases caused by trypanosomal parasites.