The paper is an accouunt of a year's work on the bionomics of Glossina morsitans in the Koalib Hills, Nuba Mountains Province, Sudan, a small area which lies 200 to 300 miles north of any other locality where G. morsitans occurs.

The fly is restricted at all times of the year to the hills, where it was found to breed in rock-shade, and is always closely associated with the Nuba inhabitants and their animals, even frequenting villages and their environs. The cattle, goats and sheep are resistant to trypanosomiasis, but all have been proved to be carriers of the disease.

Wild fauna is scarce in and near the area, and the fly is probably entirely dependent for food on the Nubas and their animals.

An experiment in direct control in a small group of hills is described. It was found impossible to reduce the fly beyond a very low minimum, but the method would be of use as a routine control in small hills.

Experiment with fly lime indicated that the shiny appearance of the lime deterred the majority of flies from alighting upon it.

Marked flies remained in the bush for ten to fifteen days during the rains, but eventually returned to the hills. Given a constant food supply there seems no reason why many parts of the bush, otherwise suitable, should not harbour fly during the rains and early dry season.

The author is indebted to Major R. G. Archibald, C.M.G., D.S.O., M.D., Director of Wellcome Tropical Research Laboratories, and to Mr. H. H. King, Government Entomologist, for much assistance and advice, also to the Provincial and District Officials for help in many ways.