An experiment was carried out in a small block of mixed bush in Tanganyika in an attempt to eliminate the two resident species of tsetse fly. Glossina morsitans Westw. and G. suynnertoni Aust., by means of bait cattle treated with DDT. Small cattle were used at the rate of 100 per square mile in herds of 12 or 13 per one-eighth of a square mile. Their numerical superiority over the large game was no more than about 3: 1.

The cattle were sprayed twice weekly. Originally a 10 per cent. suspension of the p.p′isomer was prepared from a commercial formulation of DDT. Later a laboratory-prepared emulsion (10 per cent. technical DDT) was used.

Extermination was expected with a reduction in numbers of old male tsetse of 99·9 per cent. In practice G. morsitans was reduced by 99·5 per cent. and G. swynnerloni by 92·5 per cent.

It is considered that there was sufficient immigration across the barrier clearing to account for the failure to achieve extermination of either species. It is not possible to say if extermination would have been attained in the absence of immigration.

It is considered that with the insecticidal formulations now available there is not much prospect of improving on these results in similar conditions although a great reduction in tsetse numbers is possible and might stop the carrying of disease. In the dry season, after the grass has been burnt, there is better prospect of success.