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Parasite species richness and diversity generally are considered higher in marine fish than in freshwater fish. While hypotheses have been proposed to explain these observations, they have rarely been tested, nor have alternatives been considered. Recent analyses involving the comparative method have shed some light on the problem, but advances have been limited due to the paucity of pertinent ecological information. It is suggested that productivity, diversity and food web structure of ecosystems are important determinants of parasite species richness and diversity. Productivity and diversity are higher in marine systems, and the food web more complex. A common goal in the study of parasite communities of fish has been to determine the stochastic or predictable nature of the infra- and component communities. Recent trends in aquatic ecology suggest that species composition, richness and food chain length are predictable and partially determined by the inter-relationships between area, habitat structure and productivity. Parasite infracommunity species richness is significantly higher for whole fish in marine (5.6 ± 3.2) compared to freshwater (2.8 ± 1.1) fishes, whereas intestinal infracommunities are not significantly different (2.9 ± 2.1 and 2.3 ± 1.5, respectively), though the range is higher in marine fish (0.1-7.9 and 0.4-6.5, respectively). This suggests that richness of larval parasites is higher in marine than in freshwater fish, and that marine fish serve more often as both intermediate and definitive hosts within the food web. Thus, fundamental differences in the life cycles and adaptations of parasites and the structure and function of food webs can explain the observed patterns of richness and diversity of parasites of fish in freshwater and marine habitats. Using data from North American eels (Anguilla rostrata) collected in Nova Scotia, it is shown that parasite communities are not controlled by biotic interactions, but are regionally determined due to colonization potential. Yet, there is still a degree of predictability, assuming that prevalence equals the probability of colonization. An appeal is made to better utilize replicate sampling programs, and to interpret parasitological data in an ecological framework that incorporates pertinent environmental and biological information.