Nearly 180,000 heterozygous translocated adult male Culex tarsalis Coquillett were released at an isolated field site in Kern County, California, in the spring of 1978. The release was an attempt to reduce the size of the native population, as egg rafts from females that mated with a translocated male would be bout 70% sterile. The ratio of released laboratory males to field males exceeded 10:1 for 4 weeks. Introduction of the translocation was confirmed in male progeny of 22 females, 1% of those tested from field collections made during and after the release. Tests in large outdoor cages demonstrated that mating competitiveness of the release stock had declined from .75 to .24 since 1976.