The feasibility of using thin stillage from a rice-spirit distillery as a substrate for the production of acid protease by Aspergillus niger was evaluated. A. niger THA001 was selected from six strains. The protease formed was not parallel with mycelial growth and the pattern of product is the type of mixed-growth associated The optimal initial pH and temperature were determined to be 4 and 30 degrees C, respectively. In order to enhance the formation of acid protease, the composition of stillage medium was modified by adding carbon and nitrogen sources. The addition of soybean oil (4%) caused a twofold increase in enzyme activity in shake flask cultures whereas more additions (6%) caused an inhibitory effect. High surface aeration effectively shortened the lag phase of cell growth in shake flask cultures. Aeration and agitation had a great influence on the production of acid protease. Optimal levels in a jar-fermenter culture were determined to be 1 vvm and 500 rpm. Maximum activity df 200 U ml(-1) was achieved in an 8-day fermenter culture under the optimal conditions. (C) 1998 Elsevier Science Inc.