Many streams in Southern Ontario are seriously infested with excessive seasonal growth of aquatic plants such as Cladophora and Potamogeton. A direct relationship, with a regression coefficient of 0.87, was observed between ambient P concentration in the water and P content of plant tissue in six rivers. Critical or growth controlling total P concentration of 60 ?g/L in stream water and 1.6 mg/g dry weight in plant tissue were determined. Unlike P, no significant correlation was observed between N content of plant tissue and N concentration in water. The correlation of total P with plant growth can be used to estimate the waste load which would result in maximum growth rate of Cladophora.