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Host-parasite associations are assumed to be ecologically specialized, tightly coevolved systems driven by mutual modification in which host switching is a rare phenomenon. Ecological fitting, however, increases the probability of host switching, creating incongruences between host and parasite phylogenies, when (1) specialization on a particular host resource is a shared characteristic of distantly related parasites, and (2) the resource being tracked by the parasite is widespread among many host species. We investigated the effect of ecological fitting on structuring the platyhelminth communities of anurans from a temperate forest and grassland in the United States and tropical dry and wet forests in Mexico and Costa Rica. The six communities all exhibit similar structure in terms of the genera and families inhabiting the frogs. Parasite species richness is highly correlated with the amount of time a host spends in association with aquatic habitats, a conservative aspect of both parasite and host natural history, and determined in a proximal sense by host mobility and diet breadth. The pattern of parasite genera and families within host genera across the regions examined is consistent with the prediction that ecological fitting by phylogenetically conservative species, coupled with historical accidents of speciation and dispersal, should be evidenced as a nested subset structure; the shared requirement for aquatic habitats of tadpoles provides a baseline assemblage to which other parasite taxa are added as a function of adult host association with aquatic habitats. We conclude that parasite communities are structured by both ecological fitting and coevolution (mutual modification), the relative influences of which are expected to vary among different communities and associations.