Colorimetric studies of face fly,Musca autumnalisDe Geer, vision demonstrated a close agreement between the spectral responses measured by electroretinogramic or behavioral (phototaxic) techniques. A trichromatic visual system describes colors as they appear to the flies and makes it possible to calculate which colors will contrast with each other. A mixture of ultraviolet and green radiations was calculated to be highly visible against green foliage or blue sky and was demonstrated to be attractive under field conditions.