Alkaline phosphatase activity (APA) present in the surface waters of five north central Oklahoma lakes was partitioned into four size classes by membrane filtration. Rentention of APA present in whole lakewater samples by 1.2μm (0–88 percent), 0.45μm (0–100 percent), and 0.22μm (0–100 percent) pore-size filters varied greatly between lakes and on different sample dates Collectively, the data indicated that significant lakewater APA may be present in particles less than 1.2μm in size and that dissolved APA may represent either a major or an insignificant proportion of total lakewater APA, depending on the limrological characteristics of the lake system examined.