Efforts to ameliorate the negative effects of introduced predators on native species depend on a clear understanding of the complex interactions between predator and prey. We illustrate the necessity of considering size variation of predators and prey and antipredator responses of prey, using a case study of an endangered fish, June sucker (Chasmistes liorus) and an introduced predator, white bass (Morone chrysops). Using a mathematical model, we assessed vulnerability of young June suckers to predation by a growing cohort of white bass. Additionally, we experimentally evaluated whether young white bass selectively occupy vegetated habitats (in contrast to adults) similar to those occupied by young June suckers. June suckers are most vulnerable to young white bass immediately after they hatch and vulnerability declines thereafter. They are vulnerable for some period of time under all combinations of predator and prey growth rates. Young white bass selected vegetated habitats, and this response was increased in the presence of adult white bass. These data suggest that young white bass may be a significant source of mortality for small June suckers, contrary to previous assumptions. Results of this study inform conservation actions designed to decrease the effect of predation on small June suckers, such as potential effects of adding vegetated refuge habitats, possible effects of manipulation of spawning time, and the design of predator-free grow-out areas.