Species introductions can have substantial impacts on ecological communities. The composition of communities can be described using the functional traits of community constituents, and thus trait-based approaches for quantifying community composition can represent a powerful strategy for investigating the ecological and evolutionary consequences of non-native species. Here, we report on how introduced trout species in alpine lakes of the Wind River Range (Wyoming, USA) affect zooplankton communities, a major food source for trout. We predicted that size-selective predation should shift the abundance and distribution of traits possessed by zooplankton assemblages between lakes with and without introduced trout. We sampled zooplankton in 55 lakes with fish and 33 without fish. For each zooplankton taxon, we measured multiple traits that were expected to respond to fish predation including body size, size at reproduction, reproduction method, fecundity, trophic position, and habitat choice. In lakes with fish present, there was an interspecific shift in the zooplankton community assemblage to smaller bodied species. There was also an intraspecific shift within some species resulting in a smaller body size at reproduction and less fecundity when fish were present. We also found that functional richness, functional dispersion, and functional evenness were decreased by the presence of fish. Interspecific shifts between fish and fishless lakes explain 51% of the variation between lake types, intraspecific shifts account for 3%, and covariation between the two accounted for 18% of the variation. Beta diversity analysis also showed that lakes with introduced fish are more similar to one another. Our results confirm that the introduction of fish can homogenize lakes, shift the zooplankton community assemblage, and constrain the functional space.