Samples of Madison urban runoff, precipitation from Madison and New York State were analyzed for various forms of phosphorus to estimate the algal-available fraction of each of these P forms. Urban runoff particulate P forms from Madison runoff showed acid extractable inorganic P in the range of 33 to 46% of the particulate P. Ranges for the OH- and for exchange resin extractable inorganic P were 22 to 27 and 13 to 17% of particulate P, respectively. Runoff from urban areas in the Genesee R. basin (N.Y.) showed acid, base, and resin extractable inorganic P in the ranges of 30 to 48, 18 to 30, and 11 to 25% of particulate P, respectively, in general agreement with the Madison samples. Inorganic P extracted from particulate P by resin in long-term aerobic dark incubations was similar to that extracted by the resin in short-term tests, indicating that physical and chemical rather than microbial mineralization processes were probably the key factors regulating the release of inorganic P from the runoff particles to the solution phase.