During the last decade, an increasing number of instances have been noted of tsetse flies occupying unusual, and often man-made, habitats. Recently, some aberrant populations have become important, either because they have drawn attention to new aspects of trypanosomiasis transmission or because they have necessitated modifications of established tsetse control methods.From observations made on atypical tsetse populations in Nigeria, the author has classified the various types of intra-specific behaviour patterns observed and offers an interpretation. At least 1 species, Glossina tachinoides, is normally extremely versatile ecologically but some populations are now so adapted to man-made habitats that they can probably be explained only on the basis of subspeciation. Some populations of other species of tsetse, such as G. fuscipes and G. morsitans, seem also to be involved in atypical behaviour patterns.The investigations emphasize the need to clarify the role of tsetse flies in the epidemiology of the trypanosomiases and the suitability of established tsetse control measures in areas where the pattern of vector behaviour differs from that of more conventional foci of disease transmission. Much of the African continent is rapidly changing to meet the needs of large human populations and it is therefore envisaged that many more tsetse populations will become adapted to habitats dominated by man, as the only alternative to extinction, so that the problem of trypanosomiasis may remain as great, or become greater, in the future.