The ecology of four tsetse species of the group of Glossina fusca (Wlk.) was studied near the Ugbobigha field station of the West African Institute for Trypanosomiasis Research in southern Nigeria. G. tabaniformis Westw. and G. nigrofusca Newst. occurred in Lowland Rain Forest and G. medicorum Aust. in forest outliers in Derived Savannah; G. fusca was present in both habitats. Meteorological records showed that the climate in the rain-forest was cooler and more humid than in the forest outliers where, in turn, it was more equable than in the savannah. The distribution of the four species, each with its own range of tolerance, was thought to be largely determined by the climatic conditions within the various habitats.

A fly-round, using a white Zebu ox as bait, had been established in the rain-forest in August 1956; 2,549 fusca-group flies were caught in just over three years, 83 per cent, were G. tabaniformis, 16 per cent. G. fusca and 1 per cent. G. nigrofusca. The number of flies caught on the fly-round declined during the catching period; it is suggested that this was caused by the necessity to kill all flies caught in order to identify them. This disadvantage was overcome in further experiments, in which fusca-group flies were caught from an ox tethered in the rain-forest, and in which all flies caught were marked and released; the proportion of each species in the total catch was known, although the identities of individual flies were unknown. A total of 14,577 fusca-group flies were caught once per day during 22 days in July 1957 (wet season) and 7,694 during 22 days in January 1958 (dry season). G. fusca and G. medicorum, which can be readily distinguished on external characteristics, were caught on an ox fly-round laid out in an area of riverine forest in the savannah and further studies on all species were made in a primitive fly-breeding room at Ugbobigha.

G. tabaniformis was most plentiful in the middle of the rains and scarcest during the dry season. Estimation of the numbers of G. tabaniformis and G. fusca combined, during July 1957 and January 1958, suggested that flies were approximately three times more abundant in the rains. The dry-season decrease was considered to have been caused by the adverse effects of high temperatures and low humidities. Largest numbers of G. tabaniformis were caught in those areas of the rain-forest where the host animals, especially the red river hog (Potamochoerus porcus), were least disturbed by human activity. In both wet and dry seasons there were early-morning and late-afternoon and early-evening peaks in activity of G. tabaniformis. It is incorrect to describe the species as crepuscular in habit; during July 1957 flies of a mixed population composed of approximately three-quarters of G. tabaniformis and one-quarter of G. fusca were caught at a rate of only just less than one a minute even when least active. Activity at night was negligible. Analysis of the sex ratio throughout the day showed that rather more than half of the catch of G. tabaniformis before 0700 hr. and after 1800 hr. were males. During the middle of the day females greatly outnumbered males. On the ox fly-round, 58 per cent, of the catch of G. tabaniformis were females; from the tethered ox, 65 per cent, (wet season) and 70 per cent, (dry season) of the mixed catch of G. tabaniformis and G. fusca were females. The percentage of females amongst flies that emerged from puparia of G. tabaniformis in the fly-breeding room was 57.

The numbers of G. fusca caught on the ox fly-round in the rain-forest were insufficient to provide definite evidence for seasonal fluctuations in population density, although such fluctuations probably did occur. In experiments in riverine forest, 2,547 individuals of G. fusca were caught once per day in July–August 1958 (wet season) and 304 in January–February 1959 (dry season). It is suggested that forest outliers in the savannah at Ugbobigha were a suitable habitat during the wet season but that the high temperatures and saturation deficits in the dry season approached the limit of endurance of the species and prevented larger numbers occupying the habitat; mean length of life was probably much reduced in the dry season. G. fusca in the wet season was nearly equally active throughout the day, with a suggestion of peak activity just before dark. In the severe conditions in the dry season in the riverine forest, activity was negligible between about 1200 and 1700 hr. Analysis of the sex ratio of G. fusca in the wet season showed that the proportion of males increased during the day. In the rain-forest, 44 per cent, of the examples of G. fusca caught on the fly-round were females; in the riverine forest 47 per cent, (wet season) and 40 per cent, (dry season) were females. The sexes emerged from the puparia in equal proportions.

Only 29 individuals of G. nigrofusca were caught on the ox fly-round in the rain-forest, of which only three were females.

Only 120 individuals of G. medicorum were caught in the wet-season experiment in the riverine forest and 36 in the dry season; 54 per cent, were females in the wet season and 42 per cent, in the dry season. The flies were most active early in the morning. The emergence rate (82%) from puparia of G. medicorum in the fly-breeding room was higher than for G. tabaniformis (56·5%) and G. fusca (54%).

The types of habitats occupied by the Nigerian species of the fusca group are described.