We first compared prevalences and abundances of C. formosanus infection across the four fish species to evaluate natural patterns of parasitism of this introduced parasite. This is the first known report of C. formosanus in Panama and unlike reports from elsewhere, our field comparison suggests that the parasite is not broadly distributed across the fish hosts but rather appears to be specialised on the peacock bass, a novel fish host with which the parasite does not a share a long-term co-evolutionary history. We hypothesise that the patterns of infection in the field could be driven by differences in encounter/exposure rates, susceptibility of the fishes to the parasite or the host preference of C. formosanus. To distinguish these hypotheses, we used laboratory experiments that (i) held encounter rates constant to test for differences in infection compatibility in single-species trials and (ii) tested host preference in mixed-species experiments. Our laboratory experiments corroborate field comparisons, suggesting that higher infection rates in the peacock bass are due to both greater host preference for peacock bass by C. formosanus and increased compatibility with that host compared with the other fishes. The potential specialisation by this introduced parasite on a common introduced host could provide further insight into how introduced parasites establish, integrate and potentially evolve in novel assemblages of hosts in recently expanded geographic ranges. However, additional research is needed to discern whether this potential specialisation is the result of local adaptation of the parasite to a common introduced host.