The chemical fate and biological impacts of cyfluthrin in aquatic ecosystems were investigated using mesocosms (634.7 m3) and microcosms (1.9 m3) during 1989. Ten spray drift and five soil runoff simulations were conducted. Pesticide loadings were scaled by system volume, with the same experimental design in ponds and microcosms. Aqueous cyfluthrin concentrations were similar among systems, but aqueous half-life and sediment concentrations were influenced by system scale. Biological effects (zooplankton, macroinvertebrate colonization, and aquatic insect emergence) showed parallel response patterns in both systems. Large cladocerans, mayflies, and Tanypodinae chironomid populations were reduced in the higher treatments, while oligochaetes, rotifers, and Chironominae chironomids were not reduced. Bluegill stocked in microcosms were sexually immature, while bluegill stocked in mesocosms were sexually mature resulting in large fish populations in mesocosms. Higher fish densities resulted in decreased zooplankton densities in mesocosms. Microcosms were less expensive to construct and were easier to sample. These data suggest that smaller scale systems may be cost effective alternatives for environmental effects testing.