Recombinant amidase is a 55.8 kDa enzyme from the thermophilic archaeon Sulfolobus solfataricus MT4 that catalyses the hydrolysis of aliphatic amides of 2-6 C atoms as well as many aromatic amides. Single crystals of purified amidase were obtained by the hanging-drop method at 294 K. Diffraction data for the native protein (2.55 A resolution) and a putative derivative (2.20 A) have been collected at low temperature using synchrotron radiation. The crystals belong to the rhombohedral space group R3. Structure determination by multiple isomorphous replacement is in progress. It is expected that structural information from this signatured thermostable amidase will increase our knowledge of the molecular mechanisms employed to maintain high-temperature stability in thermophilic proteins.