During the summer of 1980, an attempt was made to numerically suppress a semi-isolated population of Culex tarsalis by releasing radiosterilized males. A total of 71,016 males was collected as pupae from a productive source, Poso West, sterilized by exposure to 6 KR of gamma radiation within 24 hr of emergence and released at Breckenridge, 12.5 km east of Bakersfield in Kern County, California. The incidence of sterility in egg rafts oviposited by females collected in CO2 augmented light traps increased significantly from 2.9% prior to sterile male releases to 9.2% during the release period. The mating competitiveness of the sterile males was estimated to be 1.1 based on the proportions of sterile males among all males and sterile egg rafts among all egg rafts. Even though the radiosterilized males mated competitively, the numbers released were insufficient to numerically suppress the target population.