As species introductions can result in increased resource competition for coexisting species in the receiving ecosystems, the effects of increased exploitative competition for limited food resources from an introduced fish (Pseudorasbora parva) on a coexisting fish (Cyprinus carpio) were tested experimentally using a substitutive–additive design. Additive treatments revealed that the growth of C. carpio was significantly suppressed following the introduction of P. parva with the magnitude of growth suppression directly proportional to P. parva density and biomass. A substitutive treatment that tested for the effect of intraspecific competition revealed that when C. carpio were introduced at a similar biomass to P. parva, there was no significant difference in the extent of the suppressed growth. At the same density, however, the effect of C. carpio (higher biomass) on growth was significantly above that of P. parva (lower biomass). This was independent of the initial body sizes of the introduced fishes. Thus, the interspecific competition imposed by P. parva was only as strong as the intraspecific competition of C. carpio when present at a similar biomass.