The principle of separate release of sterile males and females into different areas, as a means of enhancing the efficiency of the sterile-male technique of insect control, has been studied using small laboratory cages. Use was made of a genetic system to separate males from females in the pupal stage of Lucilia cuprina (Wiedemann) rapidly and accurately by machine. It was found that swamping a population with sterile females leads to ineffective or noninsemination of normal females because of the limited mating capacity of males (an average of 10 matings per male). Thus in nature the separate release of sterile females could lead to a decline in population size.