The factors controlling the available P status of lakes were investigated. The influence of sediment P on the P status of the overlying water was given primary attention. Sediments from several noncalcareous and calcareous Wisconsin lakes were investigated. Sediments contained large amounts of inorganic and organic P; generally more than one-half of the inorganic P associated with a gel complex dominated by short-range order hydrated iron oxides and hydrous oxides. Measurements of exchangeability using 32P and of the nutrient status of sediment microorganisms (ATP per cell relationships) and the ability of algae inoculated into sediment-water systems to utilize sediment P as their sole source of P was investigated. Evidence based in part on comparison of 32P and inorganic phosphate measurements indicated that the Murphy and Riley (1962) colorimetric method provided valid information on the levels of dissolved inorganic P in lake waters.