How does the density of an omnivorous fish affect the structure of aquatic ecosystems? Both top-down and bottom-up processes, as well as density-dependent mechanisms acting within the fish population, could be important. To investigate these processes, I examined the direct and indirect effects of fathead minnow (Pimephales promelas) density on zooplankton communities, phytoplankton biomass (Chl a), and water quality in small eutrophic ponds; simultaneously, I examined feeding habits of fatheads and trophic relationships in these ponds by integrating stomach content and stable isotope analyses. Variations in structure and composition of zooplankton communities were associated with fish predation pressure even when effects of abiotic variables were accounted for. High fish densities resulted in dramatic decreases in macrozooplankton and increases in microzooplankton densities. In contrast, fish effects were less evident at the phytoplankton and nutrient levels. At high densities, fatheads attained the highest trophic position, indicated by $\delta\sp{15}$N, apparently as a result of increased cannibalism on eggs and larvae.