PESTAB. Bluegills (Lepomis macrochirus) were exposed under static conditions without aeration to varying concentrations of baytex, BHC, sevin, technical chlordane, copper sulfate, DDT, diazinon, dichlorvos, dieldrin, endosulfan, endrin, EPN, malathion, methoxychlor, methyl parathion, parathion, perthane, toxaphene, and/or zectran. Combinations of malathion with parathion, baytex, sevin, EPN, and perthane had greater than additive toxicity, as did parathion + diazinon, copper sulfate + parathion or sevin, parathion + methoxychlor, DDT + BHC, and methyl parathion + endrin. Similar results were obtained in rainbow trout (Salmo gairdneri). None of the other 17 combinations had more than additive toxicity, and malathion + copper sulfate had less than additive toxicity. These data indicate that further research on the interaction of pesticides in aquatic environments is urgently needed to protect fishery resources. Although none of the combinations which showed greater than additive toxicity to bluegills is recommended for use in aquatic environments, safety precautions are warranted because compounds such as malathion and sevin may be used independently in or near aquatic sites.