Mechanisms by which an introduced container-dwelling mosquito, Aedes albopictus, may cause declines in a resident container-dwelling mosquito, Aedes aegypti, in South Florida were tested using a combination of field experiments and field observations. Field experiment 1 tested which species has a competitive advantage as larvae developing in water-filled tires. Densities and availability of resources (leaf litter, which is a substrate for microorganisms fed upon by larvae) were manipulated in a factorial design. Contrary to previous laboratory experiments, A. albopictus was clearly the superior competitor in this tire environment, maintaining positive population growth at higher combined density and lower per capita resource availability than did A. aegypti. The primary determinant of success in this experiment was survivorship to adulthood, and A. aegypti only survived well in this environment when raised alone at low density, with high resource availability. Field experiment 2 tested whether this advantage for A. albopictus resulted from apparent competition mediated by shared protozoan parasites in the genus Ascogregarina. In field experiment 2, A. albopictus larvae had moderate to high levels of parasitism, but A. aegypti larvae were virtually free from Ascogregarina in all experimental tires, implying that Ascogregarina played little or no role in producing the advantage for A. albopictus in field experiment 1. Thus, apparent competition does not appear to be necessary to account for the replacement of A. aegypti by A. albopictus. As a first step toward understanding variation in the outcome of this invasion, numbers of Aedes immatures and masses of adults from field collected pupae (indicators of the intensity of competition) were compared for three sites with known histories of invasion by A. albopictus and decline of A. aegypti. Differences among sites in both number of Aedes per container and masses of adults of both species were consistent with the hypothesis that intensity of competition varies among sites, and suggest that A. aegypti persists only at sites where interspecific competition is less intense. Resource competition among larvae appears to be sufficient to account for replacement of A. aegypti by A. albopictus in suburban and rural areas of South Florida, which may have been marginal habitats for A. aegypti.