Bacillus thuringiensis serotype H-14 can be added to irrigation water for control of Psorophora columbiae in rice fields. The optimum quantity of a flowable concentrate formulation to achieve the most efficient usage was determined. Tests of three quantities (0.68, 1.89 and 5.68 liters diluted to 20.8 liters with water and dispensed at 80ml/min into irrigation water) were replicated five times. The optimum amount was 1.89 liters with an average coverage of 5.34 ha. or 0.35 liters/ha. Use of the smaller amount (0.68 liters) resulted in a reduced coverage of available flooded area. The largest amount (5.68 liters) resulted in decreased efficiency and did not disperse over a larger area than did the 1.89 liter amount. The treatment of a field with the optimum amount of 1.89 liters at each introduction point for maximum coverage would require dispensing at successive but not necessarily adjacent levee overflows as flooding progresses downfield.