Dihydropyrimidinase (EC 3.5.2.2) is the second enzyme in the reductive pyrimidine-degradation pathway and catalyses the hydrolysis of 5,6-dihydrouracil and 5,6-dihydrothymine to the corresponding N-carbamylated beta-amino acids. The recombinant enzyme from the slime mould Dictyostelium discoideum was overexpressed, purified and crystallized by the vapour-diffusion method. One crystal diffracted to better than 1.8 A resolution on a synchrotron source and was shown to belong to space group I222, with unit-cell parameters a = 84.6, b = 89.6, c = 134.9 A and one molecule in the asymmetric unit.