This study determined the acute and chronic toxicity of the organophosphate insecticide fonofos to standard freshwater aquatic organisms under laboratory conditions. Fonofos was acutely toxic to bluegill (Lepomis macrochirus), Daphnia (D. magna), and midge (Chironomous riparius) at 5.3, 2.7, and 39 micrograms/L, respectively. Three fonofos formulations (technical, 94.8% A.I.; 20G, field granular 20% A.I.; and 4E, field liquid 4#/gal A.I.) exhibited similar acute toxicities to bluegill. Exposure to fonofos delayed reproduction and decreased the intrinsic rate of increase of Daphnia during 21-d chronic exposure at the lowest tested concentration (0.08 micrograms/L). The no observable effect concentration (NOEC) for Daphnia survival was 0.42 micrograms/L; 0% survival occurred at the lowest observable effect concentration (LOEC) of 1.45 micrograms/L. The NOEC for midge emergence was 3.42 micrograms/L; only 34% emergence occurred at the LOEC of 8.24 micrograms/L. Chronic 30-d exposure of juvenile bluegills decreased growth and survival at 5.65 micrograms/L (LOEC), but no effects occurred at 2.33 micrograms/L (NOEC). The relative hazard of fonofos to aquatic life is similar to other carbamate and organophosphate corn insecticides.