Static, short-term bioassays were used to determine the effects of deltamethrin on Cyrtobagous salviniae, a biocontrol agent for giant salvinia (Salvinia molesta Mitchell). Deltamethrin is a pyrethroid insecticide that kills insects on contact and through digestion. Plastic basins containing weevils and giant salvinia were placed in riverine water and also on the edges of water bodies on the banks where they were exposed to applications of deltamethrin used in the tsetse fly (Glossina morsitans Westwood) control program. Reference control basins were placed 30 km outside the sprayed area. Weevil mortalities were determined at fixed periods after aerial application of deltamethrin. Significant mortalities of weevils with a maximum of 47% were evident up to 72 hr after exposure and were related to deltamethrin toxicity. Overall, the toxicity of deltamethrin to weevils was minimal in the field infestations based on data obtained from the field assessments conducted in the post spray periods. Keywords: aerial spraying, insecticide, nagana, salvinia weed, tsetse fly.