Single crystals of cyclohexadienyl dehydratase from Pseudomonas aeruginosa have been obtained by vapour diffusion from ammonium sulphate solution (pH 6.0) at 4°C. The crystals belong to the tetragonal space group P43212 or P41212 with a = b = 105.5 Å and c = 165.0 Å. The asymmetric unit contains at least one dimeric protein molecule with Mr = 72 kDa. The crystals diffract to 3 Å resolution and are suitable for an X-ray analysis.