Studies were made of Glossina austeni captured with sticky panels during the dry and wet season in the northern and central part of the Jozani forest on the island of Unguja, Zanzibar. The density of the fly population was significantly higher in the northern part of the forest than in the central. Female and male daily catches remained stable in time in the northern part of the forest but the density of the male fly population declined significantly in the central part of the forest at the end of the dry season. The magnitude of the catch and the sex ratio of the samples was highly affected by the trap site in both locations. Only samples in the central part of the forest were biased towards one of the sexes depending on the seasons. The age composition of the male flies was independent of locality and season but the one of female flies was influenced by the seasons in the central part of the forest. The breeding and abortion rate of flies were similar in the two locations both during the wet and dry seasons. The stability of the fly population in terms of density and composition is probably a reflection of the optimal environmental conditions in this primary forest habitat.