A thermostable alkaline protease was isolated from the Aspergillus niger ZI strain in a liquid Czapek Dox medium, containing casein (1% w/v) as the sole nitrogen source. Enzyme extract was subjected to electrophoresis in SDS-polyacrylamide gel. Two protein bands were seen on polyacrylamide gel. Active enzyme was visualized in a zymograin and protease activity exhibited a molecular mass of 68 kDa on SDS-polyacrylamide gel. The optimum pH for activity was found to be 9.0. The temperature optima of the enzyme was found to be 40 degreesC at pH 9.0 and it remained stable up to 90 degreesC, with 48.4% of the original activity retained after heat treatment at 90 degreesC for 15 min. Proteolytic activity was inhibited by PMSF, but slightly inhibited by SDS.