Of 65 IGR compounds tested in the laboratory with 4th-stage larvae of Anopheles quadrimaculatus Say, 5 had LC-90 values of less than 0.020 ppm, 14 were below 0.165 ppm, and 46 others were greater than 0.210 ppm. Four of 8 formulated IGRs tested in small field plots were effective at rates of 0.01 to 0.05 Ib/acre (11 to 56 g/ha). Two of the most promising compounds, methoprene (isopropyl 11-methoxy 3,711- trimethyl-2.4-dodecadienoate 57% (E,E) and 33%, (Z,E) and Dimilin (N-(4-chlorophenyl)-N (2,6 difluorobenzoyl) urea), were effective at rates of 0.025 and 0.02 Ib/acre (28 and 22 g/ha), respectively, against natural populations of Culex nigripalpus Theobald and Cx. salinarius Coquillett. Also, in salt-marsh mangrove habitats, Aedes taeniorhynchus (Wiedemann) was completely controlled when 0.025 Ib of Dimilin or 0.05 lb of methoprene were applied by helicopter in 5 to 10 gal (19 to 39 liters) of aqueous formulation/acre. Insecticide-resistant strains of Ae. taeniorhynchus and An. quadrimaculatus were as susceptible to these 2 compounds as were nonresistant strains of the same species.