Adynamic two-layer mass balance model for total phosphorus (TP) is developed for culturally eutrophic Onondaga Lake, NY, and tested for the May 1987 through 1990 period. The model accommodates key processes in the lake's phosphorus cycle, including settling of the particulate fraction of TP, sediment release, and vertical mass transport Model development and testing were supported by a comprehensive program of field and laboratory measurements and experiments, to determine model coefficients and external loads, and to establish temporal and vertical distributions of lake TP concentrations. The model performs well in tracking documented interannual, seasonal, and vertical distributions of TP in the lake, and thus is appropriate as a management tool to evaluate management scenarios aimed at abating the lake's cultural eutrophication problem.