The significance of Glossina morsitans submorsitans Newst. to the livestock economy of Northern Nigeria is mentioned. Differences in the ecology of the fly have been noted in the Guinea and Sudan savannah zones, respectively. In the former, its distribution is relatively widespread and diffuse, whereas in the latter it is restricted to linear strips associated with river flood plains. The area dealt with in the present work lies in the Sudan zone. The relationship of the fly to its vegetational environment is described, there being a marked concentration of almost the whole tsetse population in evergreen forest islands of relatively limited extent in the second half of the dry season. The source of food is mainly warthog, which were numerous in the area.

A focus of the fly, covering an area of seven square miles, in which the dryseason concentration sites, mainly forest islands, amounted to approximately 200 acres, was selected for an experiment. It was isolated from the main focus by a natural barrier five miles wide.

Possible eradication measures in this focus are mentioned and reasons given for adopting the application of a residual insecticide.

DDT, in the form of a 50 per cent, wettable powder as a 5 per cent, suspension of actual DDT in water, was applied, during the second half of the dry season, at an estimated rate of 20 lb. per acre to the evergreen forest islands, which had previously had paths slashed around and through them. Pneumatic knapsack sprayers were used. A preliminary trial showed that, for up to six weeks after application of the insecticide, examples of G. morsitans from unsprayed forest, placed in contact with sprayed leaves and bark, died in three hours, controls remaining alive for 24 hours. It is suggested that this good persistence may be attributable to the fact that the treated surfaces were in the shade, and that the application was made during the rainless season and at a time when the vegetation was dormant. There was a rapid decline in the population of G. morsitans after spraying. The flies persisted longest (5 weeks) in a site associated with Mitragyna inermis. One application only was given to one part of the focus and in many others one application would probably have sufficed, the second being given in most instances to control G. tachinoides Westw., which was also present, and only in two instances for the eradication of G. morsitans that persisted after the first application. One of these was the site associated with Mitragyna inermis, mentioned above. The last specimen of G. morsitans was caught five weeks after the first application, and none has been caught in the sprayed area for up to 18 months after spraying. The cost of the insecticide and labour to apply it amounted to £700 for an area of seven square miles of focus in which the dryseason concentration sites to which insecticide was applied amounted to approximately 200 acres. The final result regarding G. tachinoides was indefinite for reasons which are mentioned.