The placement of 300 laboratory-reared planarians into each of 5 catch basins in Ontario, Canada during early July produced a significant reduction of 81% in Culex larvae in treated basins over that in the non-treated basins from August 23 until September 27, 1979 and an average reduction of 74% for the entire season (July-September). In laboratory tests, Culex adults reared from 100 first instar wild larvae in 2 liters of water were reduced by over 90% by the presence of 4 or more planarians, while even with 16 planarians an average of 1.3 adults still emerged. With 80 planarians and 500, 1000 or 2000 first instar larvae in 12 liters of water, emergence of adults was reduced by an average of 80, 89 and 88%, respectively. With 2000 feral larvae, emergence was reduced even in the absence of planarians due to overcrowding.