When a volatile substance collected from male screw-worm flies, Cochliomyia hominivorax (Coquerel), was presented to unmated female flies, a distinctive behavioral response was produced. Wing fluttering and preening activities increased immediately. Soon thereafter exploratory "searching" movements were initiated, and these movements intensified until bodily contacts resembling the mating "strike" of males were noted. Females 6 days old were more responsive than younger or older flies. Mated female and virgin male flies were not affected by the substance, which appears to be olfactorily active sex pheromone.