Phytase activity was measured in soils by determining the inorganic P produced from added phytate P during 20 hrs.'' incubation at 45[degree]C in the presence of citrate at pH 5. The activity was greater when the measurement was made on the moist soil as sampled than when the soil was partly dried at room temp. before analysis. Phytase activity also changed with time after sampling. When soil treated with varying quantities of alfalfa meal was steam-sterilized to destroy the native phytase and microbial population, and then inoculated with a mixed soil flora, the amt. of phytase activity produced was proportional to the amt. of CO2 evolved from the soil during incubation. In similar soil samples inoculated with various pure cultures of bacteria or fungi, development of the organisms produced phytase activity in every case. When soils treated with organic material were adjusted to pH 5, 6, and 7 and incubated to develop the microbial population, phytase activity was greatest at pH 5 and least at pH 7.