Approximately 85,000 radiosterilized males from a newly established colony of Culex tarsalis were marked with fluorescent dust and released in a semi-isolated canyon in the arid Sierra Nevada foothills of Kern County, California, during the spring of 1981. Relative abundance and sterility were monitored in the test canyon and 2 adjacent comparison canyons. Radiosterilized males survived well, dispersed throughout the test canyon and comprised 30% of all males collected. The 11% sterility introduced into the test population after releases commenced was insufficient to suppress or delay the vernal increase in female relative abundance. Overall, radiosterilized males were uncompetitive (29%) against native males for target females. The loss of competitiveness was attributed to the onset of assortative mating related to colonization; i.e., in both outdoor cage and mark-release-recapture experiments, sterile and native males mated more frequently with females of their own genotype.