gamma-Carboxymucolactone decarboxylase (gamma-CMD; EC 4.1.1.44) catalyzes the conversion of gamma-carboxymucolactone to beta-ketoadipate enol-lactone in the beta-ketoadipate pathway, which is a key part of the degradation process of aromatic compounds in bacteria and in some eukaryotes such as fungi and yeast. gamma-CMD from the thermophilic archaeon Sulfolobus solfataricus (Ss gamma-CMD) is encoded by the pcaC gene and is composed of 139 amino-acid residues with a molecular mass of 15 945 Da. Ss gamma-CMD was crystallized and X-ray data were collected to 2.40 angstrom resolution. The crystal belonged to space group P4(3)2(1)2, with unit-cell parameters a = b = 66.66, c = 184.82 angstrom. The Matthews coefficient and solvent content were estimated to be 2.14 angstrom(3) Da(-1) and 42.6%, respectively, assuming that the asymmetric unit contained three recombinant protein molecules.