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The introduced amphipod crustaceans Gammarus pulex and G. tigrinus are displacing the native G. ducheni celticus in a number of freshwater sites in Northern Ireland. We investigated parasite and epibiont infection in populations in the Rivet Lagan and Lough Neagh where both native and invading species occur. Prevalence of the four parasites and epibionts observed was higher in the native G. d. celticus than in the invading amphipods at both field sites. In Lough Neagh. G. d. celticus individuals suffered higher burdens of the rotifer Embata parasitica and the protozoan Epistylis in comparison with the invading species. These patterns may reflect host specificity by the parasites or may result from different susceptibilities of the native and invading host species. We consider the influence of parasitism on host invasions and resulting species distributions.