The trees in about 1 sq. mile of Acacia woodland near Maun, Bechuanaland, which was shown in late 1963 to have been invaded and occupied by Glossina morsitans Westw., were sprayed with Dieldrex [20 per cent. dieldrin emulsion concentrate] between 24th February and 3rd March 1964 for the control of the fly. Searches for dead birds and other vertebrates were made on the last day of spraying and three and ten days later. A total of 109 birds (belonging to 21 species), 12 mammals, three reptiles and three fish were found dead. The findings were probably biased in favour of the larger animals, because they were relatively easy to find in the long grass and less quickly destroyed by invertebrate scavengers. Insectivorous birds predominated in the first collection and birds that are not entirely dependent on an insect diet in the last, possibly because they were attracted into the area at the beginning by a mass of dead insects that was consumed quite rapidly and because they suffered from effects that developed later, The dead fish seemed to show signs of dieldrin poisoning, but there was no evidence of general pollution of the river. The damage to wild life was probably greater than if the insecticide had been applied to tree boles only, which is the more usual way of spraying against tsetses, but it is evident that dieldrin spraying against the flies is not so innocuous to wild life as has been claimed.