Recent studies of Flathead Lake, the largest freshwater lake in the western United States, suggest that it may be experiencing the effects of nutrient enrichment. Two measures to control the amount of phosphorus entering the lake from wastewater sources have been suggested to help alleviate this change. One measure instituted in the fall of 1983 was to limit the effluent concentrations from the treatment plants in the basin to 1 mg/L total phosphorus. Plants are to be on line by November 1986. In the interim, a phosphorus detergent ban is under consideration as a second control measure. This study was undertaken to assess alternative interim wastewater control measures. It was estimated that effluent limitations on the municipal plants could reduce the wastewater phosphorus load by 66 percent while a phosphorus detergent ban would reduce the load by about 28 percent. These reductions correspond to approximately 15 percent and 6 percent, respectively, of the estimated bioavailable phosphorus loading to the lake. Because of the greater impact of effluent limitations, the four treatment plants in the basin were investigated for the feasibility of immediately implementing simultaneous chemical precipitation of phosphorus. The investigations indicated that it was feasible at the three largest plants and would require relatively small capital expenditures. Annual per capita costs for phosphorus removal at the three plants were estimated to be $5, $9, and $14/person/year and would result in a phosphorus load reduction of 15 metric tons per year to Flathead Lake. Annual costs to the consumer of a phosphate detergent ban were conservatively estimated to be $11 per household. Thus, the estimated annual costs of the two strategies are $10.23/kg P reduction for phosphorus removal at the treatment plants and $32.37/kg P reduction for a phosphate detergent ban. The study concluded that immediate implementation of chemical treatment at the municipal treatment plants rather than a phosphorus detergent ban should be pursued vigorously as a more effective means of controlling wastewater phosphorus sources.