Biological invasions constitute a major threat to biodiversity and a pressing need is to find methods that reliably predict the likelihood and degree of impact of invaders. For this, it has been proposed to compare the functional response (FR, the relationship between resource use and availability) of existing or potential invaders to that of their native counterpart on shared prey. Although this approach has proven to be effective in that impact prediction for known invaders was consistent with already reported field impacts, extrapolation of results to natural populations can be improved by including biotic and abiotic environmental variables in the experimental design. For instance, parasites dominate food webs and infection often modifies host phenotype in ways that may alter resource use. Parasites are therefore likely to alter invasive - native differences in FR, which might impair our ability to predict invasive species impacts. We compared the FR of the freshwater amphipod Dikerogammarus villosus, one of the 100 worst invasive species in Europe, to that of two non-invasive amphipods, Gammarus pulex and Echinogammarus berilloni. D. villosus has invaded Western Europe along with Cucumispora dikerogammari, a co-introduced microsporidian parasite known to alter the predatory behaviour of its host. We therefore performed the FR comparison with only non-infected specimens or with a prevalence corresponding to what is observed in natural populations. Infection changed the FR of D. villosus and the between – amphipods difference in FR. Most interestingly, depending on the FR parameter, the effect was not the same.