Designs for insecticide-impregnated screens based on combinations of two different colours were evaluated for their attractiveness to Glossina palpalis palpalis (Robineau-Desvoidy) in the Ivory Coast. Screens were tested with flanking panels of mosquito netting (flanked screens) and without such panels (simple screens), using electric nets to catch flies contacting cloth of various colours or mosquito netting. No flanked screen caught more flies than an all-blue single-coloured screen; flanked screens without blue portions generally had lower catches than those with blue portions. The majority of flies was always caught over the netting panels rather than over the cloth screens, although the proportions varied for different colours. Among simple screens, one half pthalogen blue and half ultraviolet-reflecting white caught 2·4 times as many female flies as an all-blue screen and 3·6 times as many as an all-white (ultraviolet-reflecting) screen. The basis for this synergistic effect is that blue attracts flies strongly to a position near the target (e.g. circling), while the white induces landing responses; in the blue-and-white simple screens, 77% of all tsetse landed on the white surface. Several other colour combinations in simple screens gave similar results to the blue-and-white screen for females flies, for example black-and-white, and one of the blue-and-light-blue combinations. For males, the best combination for simple screens was blue and black. In tests of different configurations of blue and ultraviolet-white on simple screens, most flies were caught using a diagonally-divided screen, the lower triangular portion being white and the upper one blue.