The author reviews recent ecological research on tsetse flies in East Africa and Northern Nigeria, particularly in connexion with the flies' sensory reactions, and stresses the importance of an accurate knowledge of their daytime and night-time resting-sites and of identifying the sources of their blood meals in order to elucidate the reservoirs of trypanosomiasis. The epidemiology of the disease is considered in the light of studies of trypanosome infections in host and fly. The control of tsetse flies must be based on the practical application of ecological knowledge by methods involving either a direct attack upon the fly (such as trapping or the use of insecticides) or an indirect attack (such as bush clearing or game destruction to eliminate the fly's habitat or food supply); these methods are dealt with in some detail. The author concludes with a discussion of modern trends in research, and a number of lines of research are suggested.