Observations were made on Glossina tabaniformis Westw., G. fusca (Wlk.), G. nigrofusca Newst. and G. medicorum Aust., which all occur near the field station of the West African Institute for Trypanosomiasis Research at Ugbobigha in southern Nigeria. Identification of these species by the use of external characters has not been found possible between the females of G. tabaniformis and G. fusca, but is fairly simple between males of these two species and between both sexes of G. nigrofusca and G. medicorum. A simple key, based on external characters, is given. When necessary, females of G. tabaniformis and G. fusca were separated by dissection. G. tabaniformis, G. fusca and G. nigrofusca usually rest in a vertical position, with the head downwards, at a height of 4–6 ft. above ground (possibly lower for G. nigrofusca), on tree trunks, climber stems, etc., having a diameter of 5 in. or less.

A fly-round was started using a Zebu bull as bait. G. tabaniformis and G. fusca were caught in good numbers, G. tabaniformis forming 83 per cent. of the catch. There appears to be a decrease in population in the late dry season (January and February) in both these species. Females comprised 56 per cent. of the total catch of G. tabaniformis and 47 per cent. of that of G. fusca.

The vegetation of the fly-round, which is typical secondary rain-forest as found in southern Nigeria, was analysed; both G. tabaniformis and G. fusca prefer old forest with an absence of a lower stratum, beneath a continuous middle stratum and good upper stratum, that is to say, where visibility beneath the trees is good. G. nigrofusca comprised only 1·1 per cent. of the catch on the fly-round, although at first it was believed to be common in the area. G. medicorum does not inhabit the main forest belt, but is confined to riverine vegetation and forest islands in savannah.

G. tabaniformis is unlikely to be of economic importance, since the rate of infection with trypanosomes is low (3·7%) and this fly is confined to the forest. G. fusca and G. medicorum are both of economic importance, since their infection rates are high (14·9 and 15·4%, respectively) and they travel out into the savannah grassland. G. nigrofusca has a yet higher infection rate (26·7%), but it is confined to the forest at Ugbobigha; where, as in Ghana, it frequents the savannah, it is potentially dangerous. Females of G. tabaniformis and G. fusca are more heavily infected than males.

Females of G. tabaniformis were examined (by dissection) for pregnancy; 82 per cent. of the females were pregnant and 4·3 per cent. unfertilised. No seasonal fluctuations in the rate were apparent.