An aquatic microcosm study (simulated aquatic field study) was conducted with the technical grade of the organophosphorus insecticide diazinon. Eighteen fiberglass tanks, each 3.2 m in diameter and 1.5 m in depth, were established with sediment and water (11.2 m 3 ) from natural ponds and stocked with 40 juvenile bluegill sunfish (Lepomis macrochirus). Diazinon was applied in aqueous solution three times at 7-d intervals. Eight loading rates were used, with two microcosms at each level plus two controls. The amounts of diazinon added during each application corresponded to theoretical concentrations from 2.0 g/L to 500 g/L. The most sensitive ecological components of the microcosms were Cladocera (zooplankton), and Pentaneurini and Ceratopogonidae (insects), which were reduced at all treatment levels. Effects on many zooplankton and macroinvertebrate taxa occurred at diazinon concentrations (timeweighted averages) of 9.2 g/L and higher. Total fish biomass was reduced at 22 g/L and higher, and fish survival was reduced at 54 g/L and higher. Odonates, some dipterans, and plants were not adversely affected by diazinon at 443 g/L, the highest concentration tested. Microcosm results were consistent with laboratory toxicity data for some taxa (e.g., cladocerans, Ephemeroptera, and bluegill sunfish), but differed substantially for others (e.g., rotifers, Chironomini, and odonates). The no-observed-effect concentration (NOEC) in the microcosms (70-d time-weighted average) was near the 10th percentile of single-species LC50 values.