A technique familiar to control system engineers, namely pole assignment, is used to design a controller for maintaining the concentration of dissolved oxygon in a polluted stream at a desired level. Artificial in-stream aeration and volumetric flow rate of effluent discharge are used as the controls. Since the model parameter values are affected by the controls, and since all the states are not available for feedback, the polo-assignment technique described here uses output feedback and minimizes the sensitivity of the polos to parameter variations. Computer simulation results are included.