The development of the Manitoba Horse Fly Trap was motivated initially by the need to control tabanid flies in spite of a lack of effective and safe insecticides. This objective could be realized only through a study of orientation behaviour in the field. Starting with the old observation that tabanids are attracted to darkcoloured objects (‘targets’), the influence of colour of target, colour of background, size, shape and movement of target were investigated. The most attractive target proved to be a glossy black (or red) sphere about 24 inches diameter, about three feet above the ground. In the course of its development the trap has provided interesting insights into visual perception of tabanids which will require physiological confirmation. The attractant principle is overwhelmingly visual. An initial surmise that thermal radiation contributes to the attraction significantly has been superseded, although a thermal stimulus may be involved in the behaviour of the flies after they reach the trap. Since only the hemato- phagous sex (females) is attracted to the trap, the behaviour involved appears to be teleologically related to host seeking activity. On arrival at the trap the insects fly in the space around the target and eventually move upward into the no-return chamber at the apex. When the trap is used for fly control the apical chamber is not needed if the interior of the trap is treated with a powerful, residual insecticide. In addition to its potential uses in conjunction with insecticides for control of tabanids, the Manitoba Horse Fly Trap can be used to study the correlation of flight activity with meteorological conditions as well as the seasonal and geographical distribution of tabanid species. The trap can be used effectively in the study of Stomoxys behaviour and control. If the sphere is replaced by a wick containing organic sulfides, blow flies can be attracted to it in considerable numbers.