Certain strains of Serratia marcescens synthesized two different types of alkaline phosphatase APase, constitutive CAPase and inducible IAPase APases, in low phosphate medium. Synthesis of the IAPase was repressed in the presence of high phosphate. Purification and separation of these electrophoretically distinct APases w as achieved by using fractional NH42S04 precipitation, adsorption on aDEAE-cellulose column and elution of enzymes by a linear sodium chloride gradient. Starch gel electrophoresis of certain fractions revealed the separation of not only IAPase from CAPase but its separation into four distinct isozymes. CAPase gave maximum enzyme activity around pH 9.5, whereas for IAPase a broad range of enzyme activity was found between pH 8.5 and 10.5. Reversible inactivation at low pH occurred for IAPase but very little with CAPase. CAPase was more thermolabile than IAPase at 95 degrees C. The two APases were found to be distinct in their kinetlc as well as immunological properties, suggesting two distinct enzyme species.