We measured the ecological effects of an organophosphorus insecticide, Guthion, in 12 tenth-acre pond mesocosms. The ponds received Guthion at five exposure levels selected to bracket the toxicity range for aquatic biota. Treatments were repeated eight times at weekly intervals in a manner simulating agricultural runoff. Guthion residues disappeared from the water nearly completely within 1 week of each application. We interpreted Guthion effects by examining exposure-response relationships to identify a response threshold (the EC0, determined by linear regression) for each taxon. Overall, fish and certain groups of insects were the most sensitive components of the ecosystem to Guthion. Other insects, as well as cladocerans and rotifers, were less sensitive, but higher Guthion exposure levels reduced their abundance. Copepods, snails, and plants were unaffected, and some of these groups increased in response to the loss of fish and invertebrate predators and competitors. Mesocosm results generally agreed with laboratory toxicity data.