Falcaustra vietnamensis n. sp. is described from the small intestine of Physignathus cocincinus from north-central Vietnam. The new species is characterized by the large male worms (20.2–28.8 mm in length and 557–724 μm in width) relative to known members of the genus, 2 sharply pointed alate spicules of equal length (1,128–1,256 μm in length), gubernaculum including 2 separate pieces, 1 ventral with a pointed distal end and 1 dorsal with a blunt distal end (164–192 μm and 155–172 μm in length, respectively), and 12 pairs of caudal papillae. Female worms are larger than male worms (24.2–34.1 mm in length and 532–735 μm in width), with the vulva situated in the posterior half of body, and elliptical eggs, 60–70 μm long by 42–47 μm wide. Falcaustra vietnamensis n. sp. represents the 38th species assigned to the genus and the third species recorded from a lizard host in the Oriental biogeographical region. Partial sequences of the 18S ribosomal RNA gene (rDNA), internal transcribed spacer regions (ITS), and cytochrome c oxidase subunit 1 (COI) are provided for the new species. The molecular phylogenetic position of the genus Falcaustra is briefly discussed.

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