Aquatic enclosures constructed in the littoral zone of a 0.72 ha pond in central Minnesota, were used to evaluate the impact of a mosquito larvicide chlorpyrifos on endemic macroinvertebrate communities. Chlorpyrifos was acutely toxic to several macroinvertebrates at a peak concentration of 17.3 micrograms/l. Midge (Chironomidae), mayfly (Caenis sp.) caddisfly (Leptocerus americanus) and damselfly (Enallagma hageni) populations were reduced or eliminated after application. However, oligochaete, snail, ostracod and clam populations increased in the treatment enclosure by the end of the experiment. The littoral enclosure design proved to be an effective technique for evaluating primary (acute) effects of a pesticide on a natural pond ecosystem, and should be readily adaptable to more complex experimental designs requiring replication of experimental units and longer-term studies.