Abstract We present the first reported crystallization of a member of the thiol-activated family of protein toxins. Perfringolysin O, a virulence factor of Clostridium perfringens, has been crystallized in two different forms by the hanging drop vapor diffusion method. In one form the toxin crystallizes with PEG 20000 in the orthorhombic space group C222 1 with cell dimensions of a = 47.8 A ̊ , b = 182.0 A ̊ and c = 175.5 A ̊ and the crystals diffract to beyond 2.5 Å resolution. In the second form the toxin crystallizes in a large variety of organic solvents including malt whisky. This crystal form belongs to the orthorhombic space group P222 1 with unit cell dimensions a = 47.1 A ̊ , b = 166.1 A ̊ and c = 214.0 A ̊ and with diffraction observed to 2.4 Å resolution.