In the Zambezi Valley of Zimbabwe, the numbers of Glossina morsitans morsitans Westw. and G. pallidipes Aust. electrocuted as they arrived near a visual target were increased five times by the release of a mixture of carbon dioxide and acetone vapour at the target. Catches declined to near the no-odour level as the odours were moved to release points 32–40 m upwind or downwind of the baits. This pattern of catches at the target changed when flies were trapped at the odour release points 4–40 m from the target, and changed when the odour was released at each of several points along the axis of the wind instead of at a single point. These changes suggest that the flies responded to a single-point release of the odours at distances of 30–60 m from the release point, that, when the flies arrived at the upwind end of a plume and discovered no visual bait, they flew upwind for a few metres and returned downwind for about 8 m and that the flies navigated effectively up a composite plume made by 2–33 separate release points, 1–8 m apart, along a line up to 30° from the mean direction of the wind. Data for Muscidae are also presented.