In an attempt made in 1966 to reduce the tsetse fly population in an experiment area of eight square miles in northern Tanganyika to a density low enough to permit safe settlement, undiluted technical fenthion (95% active ingredient) was applied three times at three-week intervals at 0.0041 gal. per acre from a Cassna 185 aircraft fitted with rotary atomisers. The total cost was £95 per sq. mile. Catches in the four weeks immediately following treatment on a flyround of 23,500 yd. sampling the area showed a reduction of over 90 per cent. when compared with those in the four weeks preceding treatment in the case of Glossina swynnertoni Aust. and G. pallidipes Aust. Five months after spraying, the reduction of the former species still exceeded 80 per cent, and that of the latter, 90 per cent. The reduction was smaller in G. morsitans Westw., the most numerous species; a fourth application would probably have improved the kill. The results are compared with those of an earlier experiment in the same area using endosulfan. Some settlement took place in the part of the area where G. swynnertoni predominated.