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Several randomization methods have been used to investigate the influence of competitive interactions in shaping parasite community structure. Marine fish parasite communities have often been regarded as unstructured assemblages with little or no resource limitation and, therefore, not prone to competitive influences. In this study, null models were used to assess the niche overlap of cestode communities of two distinct but closely related elasmobranch hosts: the round stingray, Urobatis halleri, Cooper (1863) and the skate, Leucoraja naevus, Muller and Henle (1841). Cestode species distributions were analyzed for randomness with respect to two niche axes: host size class and location within host spiral intestine. Niche overlap of cestode species was calculated for each niche type using MacArthur and Levins' and Pianka's indices, and compared to overlap values obtained from randomly generated communities. Cestodes of both host species were distributed heterogeneously among valves and host size-classes. The majority of parasite species (including all common ones) within U. halleri and all species within L. naevus had significantly nonrandom distributions with respect to at least one niche category. Cestode species pairs tended to overlap significantly less than expected in spiral intestine valves and significantly more than expected among host size classes. We conclude that cestode communities are structured deterministically in a way consistent with expectations based on competition.