ABSTRACT: In marine ecosystems, little is known about how competition with invasive fishes may affect the resource use of native predators. Throughout the western Atlantic, invasive Indo-Pacific lionfishes (Pterois spp.) are likely to compete with native mesopredators such as the graysby Cephalopholis cruentata, an ecologically similar serranid. In conjunction with a before-after-control-impact lionfish removal experiment, this study measured whether graysby population size, diet, and condition varied in relation to cohabitant lionfish biomass. Lionfish, graysby, and prey populations were surveyed and sampled along a contiguous reef ledge in Biscayne National Park, south Florida. Mesopredator diet was measured with stable isotope (δ¹³C and δ¹⁵N) and gut content analyses, and isotopic niches were used to compare patterns of inter- and intraspecific resource use diversity. The isotopic niches of graysby and lionfishes overlapped by 67%, suggesting similar population-level resource use. On sites with higher lionfish biomass, graysby isotopic niche was 34% smaller and overlapped 47% less with that of lionfishes, possibly indicating both a narrower breadth of resource use and associated interspecific niche segregation. Although gut content analyses suggested that graysby may consume less fishes on high lionfish biomass sites, prey fish populations did not vary accordingly, potentially inferring interference by lionfishes on graysby foraging behavior. However, graysby condition was not related to lionfish biomass, so the 2 species ultimately did not fit the classic definition of competitors. By discussing potential influences of lionfishes on graysby resource use, our research contributes useful information to the study of how invasive lionfishes may affect native predator communities.