A specific D-asparaginase was isolated and crystallized from Thermus aquaticus strain T351. It is present in larger amounts than the L-asparaginase. The enzyme has a molecular weight of 60 000, an isoelectric point of 4.8 and a Km of 2 mM. It has 6 disulphide bonds/molecule, and a histidine residue at the active site. It is inhibited by keto acids and by high salt concentrations.