Background/Question/Methods An organism's body shape is known to have strong impacts on its survival and reproductive success. In fishes, body shape is believed to be under multiple agents of selection, including natural selection for swimming ability and predation, and sexual selection for competition for mates. Previous studies have demonstrated the importance of water velocity on body shape, but this has typically been examined on the species-level, thus not accounting for impacts of evolutionary history. In this study, I explore the roles of habitat and phylogenetic constraints on the evolution of body size and shape in a family of teleost fish. Specifically, I examine the influence of water velocity, sex, and phylogeny on body shape in the Fundulidae family (North American killifish). The Fundulidae system is ideal for exploring these questions because there exists variation in body shape, both within- and between-species, and many of the species have populations present in both lentic (still water- lakes, ponds, etc.) and lotic (moving water-streams, rivers, etc.) habitats, thus making it possible to test for differences due to the environment (water velocity). This study uses geometric morphometrics on museum specimens to examine variation in body shape both within- and between-species. The body shape data is then mapped onto a molecular phylogeny to explore affects due to evolutionary relatedness. Results/Conclusions There seems to be a strong influence of phylogeny, with noticeable differences between the two Fundulus clades. In particular, closely related species tended to group together when examining both body depth and dorsal and anal fin positions. Also, both water velocity and sex have impacts on various aspects of body shape. Water velocity influenced body depth, with some species having deeper bodies in lentic habitats while others showing the opposite pattern. Sex influenced fin positioning, but, as with with water velocity, there was no consistent pattern across all of the species. The coupling of water velocity, sex, and phylogeny provides a comprehensive investigation of what generates variation in body shape in this family and helps our understanding of what influences a trait tightly linked to fitness.