L-asparaginase EC 3.5.1.1 was purified to homogeneity from Thermus thermophilus. The apparent molecular mass of L-asparaginase by SDS-PAGE was found to be 33 kDa, whereas by its mobility on Sephacryl S-300 superfine column was around 200 kDa, indicating that the enzyme at the native stage acts as hexamer. The purified enzyme showed a single band on acrylamide gel electrophoresis with pI = 6.0. The optimum pH was 9.2 and the Km for L-asparagine was 2.8 mM. It is a thermostable enzyme and it follows linear kinetics even at 77 degrees C. Chemical modification experiments implied the existence ofhistidyl, arginyl and a carboxylic residues located at or near active site while serine and mainly cysteine seems to be necessary for active form.