Stability of phytase enzyme from Aspergillus niger against different pH values and proteases was studied to reveal its potential as poultry feed additive. Phytase enzyme lost approximately 53, 32, 66, and 87% of its activity at pH 1, 2, 8, and 9, respectively, but the enzyme was able to recover full activity at pH 5.5 following 6 h of exposure at 37degreesC to pH as low as 3. Pre-treatment of phytase enzyme with saliva for an hour resulted in lose of enzyme activity about 10%, and similarly treatment with proteases caused about 40% decrease of activity. Addition of phytase enzyme to the low phosphorus containing diets kept the tibia ash content constant. Supplementation of diet with phytase enzyme increased availability of total phosphorus. Phytase supplementation of broiler diets also improved growth and body weights and decreased mortality.