Males of the day-flying mothLymantria disparL. (gypsy moth), while flying upwind toward a pheromone source, respond to high frequency (>15KHz) sounds by deviating sharply from the established flight course and flying away rapidly. Pheromone-stimulated males fanning their wings while walking on the substrate “freeze” momentarily in response to high-frequency sound. The puncturing of both tympana eliminates auditory behavior, whereas perforation of only one tympanum causes highly directional flight course deviations toward the perforated (silent) side. The retention of ultrasonic sensitivity in this essentially day-flying species may be explained by the selective advantage from bat predation gained by individuals active near dusk.