In this study I investigated the important ecological interactions that contribute to maintaining a striking species distributional pattern for the Enallagma damselflies (Odonata: Coenagrionidae) among lakes that do and do not support fish populations. One group of Enallagma species is found as larvae only in lakes containing fish, while the remaining Enallagma species are found as larvae only in fishless lakes. I performed observational and experimental studies to evaluate the importance of water chemistry, competition among the Enallagma species, and predators of the Enallagma species in maintaining the two discrete assemblages. The results of observational studies suggested that water chemistry could not account for the pattern of species distributions, but that differences in predator compositions and competition among the Enallagma species may. Quantitative sampling indicated that dragonfly larvae were the most abundant invertebrate predator group in both lake types, with each lake type supporting a characteristic set of dragonfly species, as in the damselflies. Fish were also potentially major predators in lakes containing fish. Also, Enallagma densities in both lake types were among the highest reported in the literature, suggesting that density—dependent competitive interactions may potentially contribute to maintaining the Enallagma species distributions. The results of laboratory and field experiments demonstrated that predation by large dragonflies in fishless lakes and predation by fish in fish—containing lakes are two major environmental factors maintaining the Enallagma species distributions. When simultaneously offered two Enallagma species in the laboratory, one from the fishless lake group and one from the fish—containing lake group, fish consumed more individuals of species from the fishless lake group, but all dragonfly species, regardless of lake type affiliation, consumed more individuals of species from the fish—containing lake group. The results of field experiments in which I transplanted Enallagma species between the lake types showed that large dragonflies in fishless lakes and fish impose much greater mortality on Enallagma species with which they do not coexist than on those with which they do. Other field experimental results indicated that density—dependent competitive interactions among the Enallagma species were only apparent in the fishless lakes, affected species from both groups similarly, and altered their growth but not their survival. The results of this study are discussed in the context of overall community organization and the consequences to the evolution of species.