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A new pinworm parasite is described from Abrocoma cinerea, a caviomorph rodent of the superfamily Octodontoidea from the Andes of Bolivia. The new species, Helminthoxys abrocomae n. sp., possesses special secretory mamelons which we consider a synapomorphy of the genus Helminthoxys. Within Helminthoxys, the closest relatives are found in octodontoid rodents: H. gigantea occurs in Octodon degus in Chile and O. bridgesi in Argentina, and H. freitasi is a parasite of Thrichomys aperoides in Brazil. H. abrocomae n. sp. differs from both other species morphometrically in relation to different parts of the body in both sexes, particularly the size of the body, spicule, gubernaculum and eggs, by the presence of a rough cuticular area around the cephalic sensory papillae and by the possession of very well-developed cervical alae which are strongly curved dorsally. H. abrocomae n. sp. is the tenth nominal species described in Helminthoxys, all of them being parasites of caviomorph rodents.