Competition between colonies of Aedes (S.) polynesiensis and Aedes albopictus in small cages, a highly artificial environment, results in elimination of A. polynesiensis. In a large walk-in cage, we simulated conditions of the habitat of these mosquitoes. Breeding containers for the larvae consisted of enamel pans, coconut shells, sections of bamboo, and earthenware pots. Adults fed on man and guinea pigs as well as on 10% sucrose solution. After A. polynesiensis increased until about 4,000 adults were emerging per week, 200 male and 200 female A. albopictus were introduced. Total weekly adult production for both species, as determined by counts of pupal skins, remained relatively constant, ranging from about 3,000 to 6,000 per week. As the proportion of A. albopictus increased, that of A. polynesiensis decreased. These proportions were 16.2% and 83.8% on week 17, 75.5% and 24.5% on week 22, and 7.9% and 92.1% on week 41. Samples of blooded adults showed the progressive decline of A. polynesiensis. After 41 weeks, A. polynesiensis had been reduced to about 5% of its maximum density, representing a control of about 95%. The essential factor in replacement of A. polynesiensis by A. albopictus is thought to be the higher reproductive rate of the latter species, which permitted it to monopolize the ecological niche provided by the large cage.