A population of Glossina palpalis (R.-D.) was studied for three years, between February 1954 and January 1957, at Ugbobigha, on the northern edge of the main rain-forest belt of southern Nigeria. A fly-round was laid out along the banks of a forested stream, running northwards from the forest belt into the savannah woodland, and was divided into four sections representing farmland, wide fringing forest, narrow fringing forest, and the edge of forest.No major seasonal movements of the fly population from one section to another could be detected; it was found that open farmland and the edge of forest with good visibility always yielded the highest catches, whereas catches inside dense forest were always low. Data from other sources showed that at all seasons of the year G. palpalis may be found in small numbers throughout the savannah woodland, even in the absence of water. This species also occurs in very small numbers within the main forest belt, and in village clearings lying within this belt.Apart from an increase in the second and third months of the rains (March–April) and a fall during the fourth and fifth months, the population tends to remain steady at a low level throughout the year. The increase in population is associated with a mean temperature of 79–80°F. (26·1–26·7°C.) and an average saturation deficit of 5·5 mb. These figures agree closely with those found to be optimal for G. palpalis by other workers. The decrease in population is associated with a falling temperature, but more especially with a mean saturation deficit that has dropped below 3 mb. These results agree closely with those found near the northern limit of the range of the species in Nigeria. The favourable period for increase during the wet season lasts for four months in northern Nigeria, but only two at Ugbobigha: this is believed to account for the much lower fly density found in the humid south.Dissections of females showed no seasonal trend in the proportion that were pregnant, nor any cessation of breeding in the heavy rains (August–September). Of 3,130 flies caught, 45·5 per cent, were females, and of the 1,040 females dissected, only 3·6 per cent, were found to be virgin; in northern Nigeria, the comparable figures were 49·1 and 4·0 per cent., respectively. At Ugbobigha, there is no seasonal trend in the percentage of females amongst flies appearing to man. Among the non-teneral males caught, the mean monthly proportion that was hungry ranged from 8 to 22 per cent., but showed no seasonal trend, whereas comparable figures from northern Nigeria ranged from 24 to 43 per cent.; this suggests that G. palpalis requires food less often in the humid conditions of Ugbobigha, or possibly that game is more numerous.Of 1,635 examples of G. palpalis dissected, 2·1 per cent, were found to have mature infections of trypanosomes; of the latter, about two-thirds belonged to the Trypanosoma vivax group and about one-third to the T. congolense group. There was no difference in the infection rates of males and females.Very few pupae were found, owing to the low fly population and the extensiveness of the breeding sites under the equable climatic conditions. The temperature of the pupal environment is considerably higher here than in northern Nigeria.The diurnal rhythm of activity of G. palpalis was investigated both in the dry season and the wet. In both, activity increases steadily between 7 and 10.30 a.m., as the temperature rises, and falls steadily between 3.30 p.m. and dusk; peak activity occurs at various times between noon and 3.30 p.m. The numbers caught before 11 a.m., expressed as a percentage of the day's catch, increases at seasons when the temperature is higher. The flies are equally active in overcast weather and full sunshine, but rain reduces activity by more than a half.