Margosan-O, an insecticide formulated from extracts of neem tree (Azadirachta indica) seed kernels, besides being toxic, also has feeding, oviposition-deterring, and growth-inhibitory effects on insects. This product, registered in the United States for ornamental plants, has been proposed for food crop use. However, little information exists on its effects on aquatic organisms. This study investigated toxicity of Margosan-O to the mosquito Culex spp., a possible target species, and to nontarget species-two crustaceans, Daphnia magna, Hyalella azteca, and a dipteran, Chironomus riparius. The 48-h EC50 value of 105 mg L-1 for Culex spp. was significantly more toxic than for C. riparius (281 mg L-1), not significantly different from D. magna (125 mg L-1) but was significantly less toxic than for H. azteca (71 mg L-1). A concentration of 20-30 mg L-1 caused growth inhibitory effects in Culex spp. and C. riparius larvae and 40 and 84 mg L-1 affected growth and reproduction in H. azteca and D. magna, respectively. Margosan-O may not be suitable for mosquito control since the concentrations required to control emergence may have some nontarget effects. Alternatively, the agricultural application of Margosan-O is also not expected to reduce the survival or produce growth and reproductive effects in nontarget aquatic organisms. However, based on estimated concentrations of less than 10 mg L-1 in adjacent shallow bodies of water and recommendations for repeated applications, there should be concern that the threshold for chronic toxicity is too narrow.