A phosphorus budget and mass balance model were developed for Lake Champlain in order to identify load reductions necessary to attain interim in-lake total phosphorus concentration criteria established in a water quality agreement between New York, Quebec, and Vermont Total phosphorus loadings were measured from 31 tributaries, 88 wastewater discharges, and direct precipitation. Mean annual tributary loadings were estimated using the FLUX program (Walker 1987). The total base year phosphorus loading rate of 647 mt·yr−1included 29% from point sources, 47% from cultural nonpoint sources, and 24% from natural sources. A mass balance model for 13 lake segments was developed and calibrated to the data using the BATHTUB program (Walker 1987). Exchange flows between lake segments were evaluated using a mass balance for chloride. The BATHTUB program error analysis procedure was used to evaluate model prediction uncertainty, based on variance estimates for all input data terms. The modeling results were used with a minimum-cost optimization procedure to determine that an overall phosphorus load reduction of 192 mt·yr−1distributed among specifically targeted lake segment watersheds will be needed to attain the in-lake phosphorus criteria.