ABSTRACT: To gain insight about how an invasive predator may influence native prey, we performed a series of experiments in aquaria to characterize and compare the prey preferences of the invasive red lionfish Pterois volitans and an ecologically similar native mesopredator, the graysby grouper Cephalopholis cruentata. Preference for native congeneric fishes, the fairy basslet Gramma loreto and blackcap basslet G. melacara, were tested. We observed behavior of predators in response to 2 individual prey consisting of cross-factored combinations of species (fairy and blackcap basslets) and size (small and large). Upon initial exposure to prey, lionfish first hunted fairy basslet and graysby first hunted blackcap basslet, with both predators initially preferring large over small fish. Overall behavior (quantified from the entire duration of observation) indicated both predators lacked a preference between basslet species based on total number of strikes and hunting time. Despite essentially identical size ranges of predators studied, graysby overall preferred large basslet across all graysby sizes, whereas the overall preference of lionfish between prey size varied with lionfish size. Importantly, the initial preferences of predators were likely least affected by the unnatural setting in aquaria. By preferentially consuming the less-preferred prey species of native graysby or by increasing predation on larger basslets, invasive lionfish may enhance coexistence between basslet species or among basslet sizes within local populations structured according to a size hierarchy. Alternatively, increased consumption of basslets may deplete local basslet populations, especially if lionfish exhibit prey switching behavior.