The regulatory properties of phytase/alkaline phosphatase purified from rat intestine were investigated. Alkaline phosphatase activity was significantly inhibited by phenylalanine, while phytase activity was not affected. Both activities were similarly inhibited by inorganic phosphate, a product of the enzyme reactions, although the modes of inhibition were different. The inhibition of phytase activity was noncompetitive, while the inhibition of alkaline phosphatase activity was competitive. Vanadate, a nonspecific inhibitor of phosphatases, strongly inhibited alkaline phosphatase activity but the inhibition of phytase activity was moderate. Phytate, the substrate for phytase, inhibited alkaline phosphatase activity biphasically. At lower concentrations inhibition was mild and competitive, but it became strong at higher concentrations. In addition, the heat stabilities of the two activities were also different with incubation at $50^{circ}C$ for 20 min, 90% of the initial alkaline phosphatase activity was lost while loss of phytase activity was negligible. These results suggest that the active sites for the two activities are not identical.