Identifying the factors that promote or preclude the evolution of resource polymorphism is essential for understanding the origins of diversity. Although such polymorphisms have long been viewed as an adaptive response to intraspecific competition, they are by no means ubiquitous, even in populations experiencing strong competition. In the present study, we examined a potentially important cost of resource polymorphism. Specifically, resource polymorphism typically entails the evolution of one or more resource-use specialists, and these specialists may suffer more from competition with other specialists than generalists would with other generalists. Using spadefoot toad tadpoles as a model system, we combined stable isotope analyses with an experiment aiming to characterize dietary differences between alternative carnivore and omnivore morphs and to assess the potential ecological consequences of any such differences. We found that carnivores and omnivores represent alternative trophic specialists and generalists, respectively. We also established that the specialist morph (carnivores) experienced greater intramorph competition than the generalist morph (omnivores). We hypothesize that the greater intramorph competition faced by specialists stems ultimately from functional limitations associated with trophic specialization, which prevent specialists from switching to alternative resources when their resource is depleted. These costs may even preclude the evolution of distinct resource-use specialists, and hence resource polymorphism, in certain populations. © 2012 The Linnean Society of London, Biological Journal of the Linnean Society, 2012, ••, ••–••.