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This report describes a method and its application in analyzing water quality trends at long-term Water Quality Monitoring Network stations in Ontario. The multiplicative form of the (U.S.) National Sanitation Foundation method, with some modifications, is used to aggregate data for seven water quality variables (dissolved oxygen, total coliforms, 5-day biochemical oxygen demand, total phosphorus, nitrate, total soli ds and turbidity) into a single number for each year for the period 1964 to 1979. This number, referred to as an annual water quality rating, is used to facilitate comparisons of water quality with time and among stations. Of the 169 long-term stations selected for study, 95% (161 stations) show improving or no change in quality. Only 5% (8 stations) show signs of deteriorating quality. Abatement measures relating to these water quality trends are discussed. Two significant measures relate to the reduction of phosphorus in effluents from municipal water pollution control plants through (1) the reduction of phosphorus by the reformulation of laundry detergents after 1970, and (2) the implementation of phosphorus removal at municipal water pollution control plants after 1972. Relative to 1979 data, broad regional (area-wide) differences are apparent among stations when grouped on the basis of their annual water quality rating numbers. These differences among station groups appear to be related to the population distribution and geologic environments within the Province.