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Didelphoxyuris thylamisis n. gen., n. sp. is described from the caecum of Thylamys elegans (Waterhouse, 1839) (Marsupialia: Didelphidae) collected in the eastern region of the Andes of Bolivia. Didelphoxyuris thylamisis n. sp. differs from the only pinworm described from marsupials in the Neotropics (Neohilgertia venusti Navone, Suriano et Pujol, 1990) in having only three oesophageal teeth, non-operculated eggs, females that are didelphic, and males that possess no preanal papillae. Several other species of pinworms have been described from marsupials in Australia, but all are characterized by possessing a buccal capsule that is strongly cuticularized with inter-radial lamellae. These structures are lacking in D. thylamisis. Didelphoxyuris n. gen. is characterized by a mouth opening into a depression and lateral alae composed of two lengthwise crests. All males possess an area rugosa composed of a ventral sagittal crest, a caudal extremity truncated at the level of the cloaca, and four pairs of genital papillae (two pairs lateral adanal and sessile, one pair just posterior to spicule aperture, and last pair at posterior extremity and pedunculated). All females possess an opisthodelphic uterus, a reflexed ovary and a thick and laterally enlarged cephalic vesicle containing a complex reticulated network of rounded confluent vesicles.