The thermostable amylopullulanase from Pyrococcus woesei was crystallized. Crystals, suitable for a crystallographic analysis up to a size of 0.6 mm in their longest dimension, have been obtained by the vapor diffusion method in a solution containing polyethyleneglycol 4000 (PEG 4000), isopropanol, and Tris/Cl- buffer pH 7.5. Crystals grown under these conditions form hexagonal rods and diffract to a maximum resolution of 3 A. The crystals belong to the trigonal lattice type with the spacegroup P3(1)21 or P3(2)21, respectively, have the cell dimensions a = b = 96.8 A, c = 196.2 A, alpha = beta = 90 degrees, gamma = 120 degrees. The crystals have a theoretical packing density of 2.7 A3/Da, assuming one molecule with a molecule weight of 88.8 kDa in the asymmetric unit. Furthermore the self-rotation analysis of the dataset revealed only crystallographic symmetries. The merged native data of two crystals resulted in a 88% complete dataset.