In an experiment carried out in 1963 on the control of Glossina swynnertoni Aust. infesting open bushland in northern Tanganyika, 3 per cent, dieldrin or endosulfan was applied to certain of the resting sites of the flies. These had been determined by previous observations and consisted of the undersides of tree branches between 4 and 9 ft. from the ground, with diameters of 1 to 4 in. and inclined less than 35° from the horizontal. Eradication was achieved in an area of 35 sq. miles at a cost of £42 per sq. mile. In a subsidiary experiment, G. pallidipes Aust. infesting the narrow forest bordering a seasonal river was attacked by spraying the vegetation on the river bank up to a height of eight feet. Complete eradication was not attained, due, it is thought, to removal of the insecticide by heavy rain, so that some newly emerged flies escaped exposure to a lethal deposit. The flies were, however, eliminated from the greater part of the river at a cost of £12.5 per mile.