ABSTRACT. – Empirical parasitological surveys typically report multiple parasite species infections in hosts. Among the different factors that affect parasite communities within a host individual or population, interspecific interactions are of fundamental significance. Dependence of parasite aggregation on interspecific competition in parasite communities has been documented for several host-parasite systems. In this study we tested the effect of community size on parasite aggregation in native and introduced fish populations of two species of grey mullets (Liza haematocheilus and Mugil cephalus). We investigated the relationships between community heterogeneity and parameters characterizing species richness and both similar and different patterns for invasive and native hosts were revealed. The degree of aggregation of parasites decreased with parasite community size, except for monogeneans from the invasive host, thereby supporting findings of previous studies. Monogenean parasites, which were introduced in the new area together with their host, showed a positive relationship between the degree of aggregation and parasite community size. This suggests unregulated interactions in the introduced hostmonogeneans system. The absence in the new host range of at least two monogenean species from the native range may have disturbed the evolutionary interaction established among them, which could have resulted in decreased interspecific competition. Multiple regression analysis showed that both species-rich communities and host introduction have a negative effect on species aggregation in parasite communities. Differences in helminth species richness, abundance and aggregation between native and invasive hosts support the idea that interspecific interaction is predominantly mediated by the host defense system.