Experiments were carried out in Kenya to find whether insecticidal smokes from “Swingfog” machines would remove tsetse flies, in this instance almost all Glossina longipennis Corti, carried by trains. Fogging was done by two operators, each with a machine, starting from opposite ends of trains halted at a station twenty miles inside a fly-infested area and assessment was by routine catches at a station twenty miles outside it. It was found that fogging reduced the number of flies found on trains by 60 to 70 per cent. As the trains had to pass through a light fly infestation after fogging, the real kill is likely to have been higher. The cost of such partial reduction is believed to be considerably higher than would be that of therapeutic control of trypanosomiasis in the affected region.