Virgin female moths of the fall armyworm, Spodoptera frugiperda (J. E. Smith), produce a pheromone within the last abdominal segment that excites males sexually. The biologically active material was purified by thin-layer chromatography. The response elicited by a fixed dose of pheromone was greater in older males. Males responded to as little as 0.02×;10−4 female moth equivalent. The pheromone was not present in very young females, but increased rapidly 18 hours after eclosion, at which time the males become receptive to the stimulus.