A propeller pump was operated for 120 days on a lake of40 ha surface area in north-central Oklahoma in the summer of 1975 to accomplish artificial destratification. The pump created a downf/ow of welloxygenated surface water by means ofa 1.82 m diameter propeller located 1.8 m below the lake's surface. The pump produced a flow of 1.72 m3/sec, equivalent to 12.9% of total lake volume per day, at 17 rpm with a 1.0 h.p. electric motor. Four days of pumping eliminated thermal stratification, it raised the temperature of the hypolimnetic water 9.5C, but increased surface temperature less than re. Thereafter the entire water column remained isothermal (27-29°C) during the summer. Dissolved oxygen at 5 m increased from 0.2 to 4.3 mg/ after the first day of pumping; thereafter DO levels at 5 and 9 m depths were above levels observed in 1973 and 1974; in mid-July 100% of the lake's volume contained more than 5 mg/l DO. BOD levels averaged 2.3 mg/l before pumping but 1.2 mg/l after pumping began. Turbidity did not change substantially with pumping but vertically the variation at 1, 5 and 9 m depths was reduced.