Physico-chemical studies of insecticide sprays applied by helicopter have been undertaken during the course of the WHO Phase II aerial spraying trials for riverineGlossinacontrol in Upper Volta during the 1977/78 dry season. A Bell 47G-4A helicopter was fitted with electrically operated rotary atomisers and the equipment was calibrated to apply insecticides at very low dosages (endosulfan 100 g/ha and deltamethrin 12.5 g/ha). This was achieved by the accurate metering of insecticide quantities to the atomisers and employing a technique of application which produced very small droplets from atomisers located along a boom on the port side of the helicopter. This method of installation allows the downwash of the main rotor of the helicopter to direct the spray droplets more effectively into the riverine canopy and is called the ‘unilateral’ technique. Distribution of insecticide deposits close to the ground during the morning were markedly different from the evening in open savanna, but not so in the riverine forest. There was evidence that droplets of approx 80 μm vmd penetrated more effectively than droplets of 150 μm vmd but a comparative evaluation of endosulfan applied at equivalent dosage rates (100 g/ha) and at these droplet sizes indicated both to be equally effective in reducingG. tachinoidesduring the first two weeks following the applications. Deltamethrin applied as a fine spray at a dosage rate of 12.5 g/ha gave similar reductions ofG. tachinoides. Studies of spray deposits within the habitat of the fly close to the ground and near the river's edge suggested that there was insufficient insecticide on the target surfaces to give a persistent deposit. However, there is evidence that deposits increased with increasing height, therefore the incidence of movement of tsetse flies during the night to resting sites within the upper parts of the riverine forest canopy may have an important bearing on their control. The ability of aerial sprays to penetrate the riverine forest canopy is of importance in the different vegetation zones and is of particular relevance in the more densely forested areas of the Southern Guinea Savanna zone where studies have yet to take place.