Repeated treatments of the insect growth regulator (IGR), Altosid® EC4(isopropyl 11-methoxy-3,7,11-trimethyl-2,4-dodecadienoate), at the rate of 0.1 ppm (0.27 lb/acre) to experimental ponds reduced abundance of several arthropod prey and predator species. Because of the size differential among affected species, the altered community biomass reflected the impact of Altosid more profoundly than abundance. Pupae of the mosquito,Culiseta inornata(Williston), were devastated by the IGR, representing large potential losses of biomass during the height of the mosquito breeding season. Chironomid larvae were reduced twofold by the treatment, and their importance to the biomass increased with rising spring water temperature. Preliminary laboratory and pond assays determined that Altosid induced mortality in early and late instars of the mayfly,Callibaetis pacificusSeeman. During colder winter months mayflies were eliminated from ponds under repeated treatment but populations in check ponds remained low and contribution to total biomass was slight. Rising water temperatures reduced IGR impact on the mayfly population. The ostracod,Cyprinotussp., was a major prey component and was not affected by treatment. A major predator, the larval dytiscid beetle,Laccophilussp., was eliminated from the treated ponds. This loss represented 84% of the predator biomass during one period. Odonata naiads comprised the 2nd major group of predators throughout the study. These naiads preyed heavily on mosquitoes and ostracods and were not affected by the IGR.