The fate of esfenvalerate was investigated by sampling and chemical analysis after spraying of an artificial pond (25 g a.i./ha) and in the laboratory with [C-14]esfenvalerate by trapping of (CO2)-C-14 and fractionation of the sediment. The effects were investigated on pelagic communities in enclosures in a natural lake and in the laboratory on surface (Cymatia coleoptrata) and sediment (Chironomus riparius) insects. The latter were used in sediment-plus-water and in water-only tests, measuring effects on emergence and mortality. The measurements in the artificial pond indicated exposure concentrations in the surface microlayer, water column, and sediment of 0.4 mug/L. 0.05 mug/L, and 9 mug/kg dry weight, respectively, two weeks after application. The degradation studies showed a limited mineralization (26.5%) of [chlorophenyl-C-14]esfenvalerate during 112 d. Part of the substance was transformed to water-soluble compounds (18.1%) or compounds attached to fulvic acids (26.2%), humic acids (14.28), or nonextractable sediment constituents (8.8%). The formulated product Sumi-Alpha 5 FW (R) caused 100% mortality to Cymatia coleoptrata after surface application of 0.13 g a.i./ha. Effects on zooplankton were recorded at 0.005 mug/L of esfenvalerate. The 96-h median lethal concentration for first-instar larvae of Chironomus riparius was 0.13 mug/L, whereas the delayed emergence lowest-observed-effect concentration was 0.8 mug/L.