Botulinum neurotoxin serotype A was isolated from liquid culture of Clostridium botulinum. The pure Mr approximately 150,000 neurotoxin, composed of Mr approximately 50,000 light and Mr approximately 100,000 heavy chains, has been crystallized in three different crystal morphologies; all three have the same crystal form. The most suitable crystal form for X-ray analysis are bipyrimidal and crystallize in the hexagonal space group P3(1)21 (or P3(2)21) with one dimer per asymmetric unit. The unit cell dimensions are a = b = 170.5 A, c = 161.7 A. The crystals diffract to 3 A resolution.