The composition of animal communities developing from planktonic larvae in aquariums containing sand and flowing estuarine water was altered in the presence of the carbamate insecticide Sevin (carbaryl). Treatments were control and concentrations of Sevin that averaged 1.1, 11.1, and 103 micrograms/l; each treatment was replicated 8 times. Animals that colonized aquarium sand were collected in a 1-mm mesh sieve after 10 wk of exposure. Mollusks, arthropods, annelids, and nemerteans were the numerically dominant phyla. The average number of species per aquarium was significantly less in aquariums containing 11.1 or 103 micrograms/l than in those containing 1.1 microgrms or in control aquariums. The abundant clam Ensis minor grew significantly less in length at the higher concentrations of Sevin. The amphipod Corophium acherusicum was particularly affected; significantly fewer were found at all concentrations than in the control aquariums. A marked increase in the abundance of the annelid Polydora ligni in aquariums containing 103 micrograms/l corresponded to a marked decrease in the number of other annelids and to a significant absence of nemerteans.