Application of the organophosphorus insecticide temephos to a natural pond in central Minnesota was followed by reduction within 24 hr in all cladocerans, inDiaptomus leptopus and inChaoborus americanus, and increases in cyclopoid copepods, copepod nauplii and the rotiferKeratella cochlearis. Daphnia pulex that reappeared 35 days post-application were ex-ephippial. After application, reproduction of cladocerans andDiaptomus was markedly reduced compared to the previous year, an effect attributed to temephos.Daphnia population density was strongly reduced into the fall season, long after the spring applications, compared with both a reference pond's and the previous season's populations. On-site bioassays demonstrated 24-hr mortalities ofDaphnia andChaoborus that were comparable to the population decreases in the pond. Laboratory toxicity tests showedDaphnia was the most sensitive followed byChaoborus andDiaptomus. Comparable results were seen in the pond population changes,in situ bioassays and laboratory toxicity data forDaphnia and forChaoborus, but sensitivities in the pond were somewhat greater than in the laboratory. Risk assessment research calls for long-term field reproduction analysis to avoid a false “recovery” report, combined with laboratory life cycle analysis and short acute exposure tests.