Wheat bran, soybean meal, corn flour and the combinations of these individual substrates with nutritive supplements containing 1% casein hydrolysate, 0.2% CaCl22H2O, 0.4% (NH4)2SO4, 0.02% MgSO47H2O, 0.05% KH2PO4 and0.04% K2HPO4 were evaluated to select an optimal medium in solid state fermentation (SSF) to produce extracellular phytase from a bacterial strain, Bacillus sp. T4. The combination of corn flour with nutritive supplements resulted in best phytase synthesis and was used for further SSF explorations. Maximum phytase production (20787±39 U/g) was observed at a growth period of 84 h, 55.5% moisture content, and 4% inoculum density. Optimum pH for phytase production was 7.0. Enzyme activity was enhanced (P