A bacterial strain producing high level of an extracellular phytase was isolated from cooked rice and identified as a strain of Bacillus sp. and designated as Bacillus sp. KHU-10. Optimum culture conditions were investigated for the maximum productivity of phytase by Bacillus sp. KHU-10. 1.0% Maltose and 1.0% peptone with 0.5% beef extract were the best carbon source and nitrogen source, respectively. The addition of $CaCl_2$, stimulated the enzyme productivity with concentration between 0.01% and 0.2%, in the medium. Although sodium phosphate increased the cell mass, the enzyme activity decreased. Calcium phytate and wheat bran containing phytate did not enhance the enzyme production. Under the optimum medium, the production of the phytase reached the highest level of 0.2 unit/ml after 4 days of incubation.