The toxicity and persistence under natural field conditions of deposits of dieldrin and isobenzan, derived from dilutions of emulsion concentrates, on the leaves of riverine vegetation in the Northern Guinea Savannah vegetation zone of Nigeria were studied by a bioassay method, using teneral females of Glossina palpalis (R.-D.) as test insects. Although deposits of both insecticides were extremely toxic when first sprayed, neither product was found to have a long residual action, and isobenzan was found to be approximately half as efficient as dieldrin. It is suggested that because of its poor residual action isobenzan will be little used in tsetse control campaigns in areas similar to that in which the investigations were performed.