Average annual total phosphorus loads to Lake Simcoe are estimated at 100 metric tons. Urban sources of phosphorus such as water pollution control facilities and urban stormwater runoff account for approximately 30% of the total loading. Rural sources such as runoff from agricultural lands, leachate from septic systems, atmospheric deposition and soil erosion contribute the remaining 70%. A target load of 75 metric tons per year has been proposed to ensure an end-of-summer deep water dissolved oxygen concentration of 5.0 mg/L. To achieve the target load, it is necessary to reduce the export of phosphorus from the major controllable sources by 25 metric tons per year. Remedial measures and phosphorus control options to reduce point and non-point sources include controlling agricultural runoff, and erosion and urban stormwater runoff. Several management strategies have been evaluated using a GIS-assisted computer model called HYDROSIM which predicts changes in phosphorus loading in response to implementation of remedial measures within the Lake Simcoe watershed. Management scenarios examined the effectiveness of remedial options based on present and future conditions to ensure that the target load is achievable and sustainable. Modelling results indicate that the most substantial loading reductions can be achieved by controlling urban stormwater runoff and cropland soil erosion. Based on the implementation of an aggressive management program, the target load of 75 metric tons per year is considered to be achievable and can be maintained into the future.