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ABSTRACT: A 2-year, seasonal, parasitological study of 1435 fish, belonging to 4 species of native fishes and 7 species of non-native fishes from the lower Little Colorado River (LCR) and tributary creeks, Grand Canyon, Arizona, yielded 17 (possibly 18) species of parasites. These comprised 1 myxozoan (Henneguya exilis), 2 copepods (Ergasilus arthrosis and Lernaea cyprinacea), 1 acarine (Oribatida gen. sp.), 1 piscicolid leech (Myzobdella lugubris), 3 (possibly 4) monogeneans ( Gyrodactylus hoffmani, Gyrodactylus sp., Dactylogyrus extensus, and Ligictaluridus floridanus), 4 nematodes (Contracacecum sp., Eustrongylides sp. Rhabdochona sp., Truttaedacnitis truttae), 3 cestodes (Bothriocephalus acheilognathi, Corallobothrium fimbriatum and Megathylacoides giganteum), and 2 trematodes (Ornithodiplostomum sp., Posthodiplostomum sp.). Of these, Rhabdochona sp. is the only adult parasite native to the LCR. Infection intensities (worm burden) of Ornithodiplostomum sp and B. acheilognathi were positively correlated with length of humpback chub, Gila cypha. Adult helminths showed a high degree of host specificity, the exception being B. acheilognathi, which was recovered from all fish species examined but was mos