Citrobacter braakii YH-15 produced an intracellular phytase which was purified 12 800 fold to homogeneity with the specific activity of 3 457 units mg(-1), which is 1.9 times higher than E. coli phytase previously recorded as having the highest specific activity. Its molecular weight was 47 kDa by SDS-PAGE gel. Enzyme activity was optimal at pH 4 and at 50 degreesC. The K-m value for sodium phytate was 0.46 mM with a V-max 6 027 U mg(-1). The phytase was resistant to proteases such as trypsin, pepsin, papain, pancreatin, and elastase.