Despite being one of the most prevalent forms of consumerism in ecological communities, parasitism has largely been excluded from food-web models. Stable isotope analysis of consumers and their diets has been widely used in the study of food-webs for decades. However, the amount of information regarding parasite stable isotope ecology is limited, restricting the ability of ecologists to use stable isotope analysis to study parasites in food-webs. This study took advantage of distinct differences in the feeding ecology and trophic position of different species of fish known to host the same common micropredatory gnathiid isopod, to study the effects of host stable isotope ecology on that of the associated micropredator. Blood engorged juvenile gnathiids were in most cases indistinguishable from their hosts’ blood, but significant isotope discrimination was observed for adults. Adult males were generally lower in δ13C and δ15N than host blood whereas host specific isotopic discrimination for females varied among the different host species.