When male pupae of Culex quinquefasciatus Say were exposed to 5 chemosterilants, 3 compounds caused over 95% sterility. Males exposed to 9 or 10 kR of gamma irradiation as pupae were over 94% sterile. The chemosterilants and the irradiation reduced mating competitiveness in most tests. Male pupae irradiated 22 to 26 hr after pupation had higher levels of sterility and less mortality than those irradiated 4 to 8 hr after pupation. The importance of both sterility and mating competitiveness in a sterile male release program was emphasized by theoretical calculations showing that fully sterile and competitive males released into a population of on million males and one million females at a 19:1 ratio would reduce the population to less than one individual in 5 generations. However, when males were 98% sterile but only 50% competitive, it would require 8 generations, and when males are 89% sterile and 90% competitive, it would take 22 generations to reduce the population to less than 1 individual.