The cultural conditions for acid protease production by Aspergillus niger was optimised using factorial design experiments and one factor-at-a time approach. In the production medium casein served as substrate and protease activity was measured in terms of tyrosine yield. The yield was further improved through UV mutation. Tyrosine yield amounted to 29.22 mg / g on casein substrate. Protease from this microbial strain was mesophilic. The enzyme was stable over a wide temperature range (30 to 45 °C) with maximum activity at 40 °C and showed better activity under acidic conditions. Also the extracted protease exhibited good tolerance towards organic solvents, surfactants and showed an increase in activity in presence of Ca2+. UV mutant showed 37.2 % increase in yield over the natural selectant and had better pH and thermostability. SDS PAGE showed 27.5 kDa as molecular size of the enzyme. This was further confirmed by zymography and gel filtration (Sephadex G-50). An improved enzyme yield and a better pH and thermal stability were achieved by using a combination of cultural condition optimization and mutation technique.