Male Culex pipiens fatigans were sterilized with thiotepa and released at a rate of 150,000-300,000 per day for five-and-a-half months into a village near Delhi around which a zone, 3km wide, was maintained free of adult C.p. fatigans production by the application of larvicides. The mosquitos for release were marked and the ratio of marked (sterile) to unmarked (wild) males was always greater than 24 : 1. In view of the known level of mating competitiveness of the sterile males a very high proportion of the matings in the village were expected to be by sterile males. Data from egg rafts collected from captured unmarked (wild) females proved that a high level of sterility was introduced into the wild population, for a period of about three weeks, but in the preceding and succeeding months the level of sterility was low. Evidence from several sources indicated that, despite the surrounding breeding-free zone, fertile inseminated females were infiltrating into the experimental village. Comparison of the density of the wild population in the experimental village with two untreated villages suggested that the period of high sterility might have caused some degree of temporary population reduction, but the evidence was not conclusive.