Many West African mosquitoes fly at low levels when crossing open country. Experiments were therefore conducted in the Gambia on the effect of high fences on their dispersal. In the first experiment, a circular mosquito-proof fence 2·9 m high and with a radius of 18 m was constructed. Catches inside on human bait of Mansonia spp. were about 60% lower than catches outside the fence, whereas catches of Anopheles spp. in the two sites were not significantly different. In unbaited suction traps, catches of Mansoniainside the fence were significantly lower in traps 0 and 0· m above the ground, but not at 1 to 3 m, than in traps at similar levels outside. This suggested that, after flying over the fence, mosquitoes had not resumed normal ffight levels by the time they were trapped in the centre of the circle. In a second experiment, a much larger fence was erected, 6 m high and with a radius of 65 m. Catches were conducted in experimental huts in the centre of the enclosed area and at a distance of 70 m outside it when the fence was erected and again after its removal. Catches of Mansonia spp. on human bait in the hut inside were 46% of the total catch in the two huts when the fence was erected and 48% after its removal; catches of Anopheles spp. (A. gambiae Giles and A. funestus Giles) were 43% with, and 53% without, the fence. In calf-baited light-traps, catches of Mansonia were 41 % of the total with, and 38% without, the fence, and those of Anopheles 58 and 44%. Thus the presence of the 6·m fence had no significant effect on the density of mosquitoes in the centre of the enclosed area, and it is concluded that mosquitoes were flying freely over it. Due to the prevailing low winds in the area, passive transport of mosquitoes over the fence by the wind was not thought to be important.