The submerged cultures of Aspergillus nidulans had optimal growth and protease production at 37 degrees C and within 6 days of incubation. A rapid drop in pH of the growth medium from 6.9 to 4.8 and a subsequent gradual rise was recorded with the period of incubation. The acid-protease produced was purified by a combination of ethanolic precipitation, ultrafiltration and fractionation on DEAE-cellulose and Sephadex G-200. A single peak showing protease activity was subsequently obtained with a 16-fold increase in specific activity and a recovery value of 36%. The purified enzyme had optimal activity on casein and gelatin at pH 5.4 and a temperature of 40 degrees C.