A technique, based on the release and recapture of marked females, was used to measure the mating competitiveness of sterile males of Culex pipiens fatigans Wied. in villages in Delhi Union Territory, India. Experiments were carried out with chemosterilised and cytoplasmically incompatible translocated (IS-31B) males simultaneously with prolonged release trials with these two types of male. The fertile males in the tests were the indigenous population and the females used were in most cases of wild origin. With each type of male, one test was conducted with a moderate (ca. 13:1) ratio of released to wild males and the other tests used high (ca. 40:1) ratios. The estimated mean mating competitiveness of IS-31B males was somewhat higher than that of chemosterilised males, but the difference was found to be non-significant using a specially devised method of testing statistical significance. In the four experiments 77–94% of the egg rafts laid by recaptured females were sterile, indicating that both types of sterile male were sufficiently competitive to induce a high level of egg sterility in an isolated wild population.