An aquatic, ecosystem level study with the insecticide sulprofos was conducted using twelve 470-m3 earthen ponds and twelve 11.0-m3 fiberglass tanks. For each test system, the initial benthic and limnetic communities came from the same sources. The ponds and tanks were stocked with adult and juvenile bluegill sunfish, respectively. Physical and chemical water quality parameters were similar in the ponds and tanks except that dissolved oxygen, hardness, and conductivity tended to be higher in the tanks while total suspended solids were higher in the ponds. Sulprofos concentrations in the water were usually slightly lower in the tanks. Sulprofos concentrations in the sediment were lower in the ponds. Phytoplankton chlorophyll a concentrations in the tanks and ponds were very similar both in terms of magnitude and variation. The responses to sulprofos of the zooplankton, benthic macroinvertebrates, and fish in the ponds and tanks were not always the same. The differences appeared to be related to differences in exposure to sulprofos or the differing abundance of bluegill. These results suggest that fiberglass tanks of this size could be cost-effective precursors to earthen ponds for pesticide registration tests.