Abstract – Nonnative parasites have the potential to detrimentally affect naïve hosts, resulting in negative consequences for their growth, condition and energetics. Here, the effect of the introduced parasitic nematode Philometroides sanguineus on crucian carp Carassius carassius populations in England was investigated. Populations of C. carassius populations are increasingly spatially restricted in England and under increasing threat from habitat loss and hybridisation. Parasite prevalence across 6 infected populations was <27% and, generally, there was no significant relationship between levels of infection and fish length and age. Parasite abundance ranged between 1 and 8 nematodes and was not significantly related to fish length and age. Comparison of the growth, body weight and condition, and energy reserves between infected and noninfected C. carassius revealed infection did not incur significant detrimental impacts on these parameters. Whilst this suggests that infection had only minimal impacts on the examined host fish, this may have been a consequence of a low proportion of fish <100 mm in samples (i.e., size-selective effects) and some tests did suffer from low statistical power because of, for example, unbalanced sample sizes. It does, however, suggest that P. sanguineus may not be a major threat to the status of these C. carassius populations and infection by introduced parasites may not always incur significant impacts in naïve fishes.