In an attempt to understand the significance of interspecific interactions to the evolution of parameters of population growth, aspects of larval success for two geographic strains of the treehole mosquito, Aedes triseriatus, are examined. A strain from Illinois, beyond the known range of the predatory mosquito larva Toxorhynchites rutilus, exhibits success in single-species laboratory cultures which is consistently superior to that of a strain collected in North Carolina within the predator's range, at a spectrum of food levels. This result, based on observations of wing length, survivorship, development time and a composite index of success, permits the conclusion that the Illinois strain has the higher capacity for growth in conditions of high and low resource availability. However, the functional response relationships between Toxorhynchites feeding behavior and density of A. triseriatus larvae suggest that the North Carolina strain is more difficult for the predator to eat than the Illinois strain. A potential competitor, Orthopodomyia signifera, was least successful in the presence of the North Carolina strain of Aedes triseriatus according to the criteria of female wing length and a composite index. These findings suggest that growth rates and single species competitive ability may be reduced in response to the evolution of effective interactions with other species.