When the response of Rhagoletis cerasi (L.) to 27 colors or shades of enamel paint was assayed under orchard conditions, more flies were captured on stickycoated rectangles painted yellow or dark tints of yellow than on any other. Daylight fluorescent yellow captured even more flies than yellow enamel. Evidence indicates that the flies’ response to yellow was one of positive attraction and true color discrimination. The biological basis of fly response to yellow may lie in the fact that yellow is the most intensely reflective color in the general part of the spectrum where green leaves (on which flies find food) reflect their most light, 500-600 mµ.