Simple SummaryA new spirurid nematode,Protospirura canariensisn. sp., a parasite of the black rat in El Hierro (Canary Islands, Spain), was described by means of light and scanning electron microscopy. The morphological characteristics of the new species and the existingProtospiruraspecies, as well as their host species and geographical distributions, were useful criteria in distinguishingP. canariensisn. sp. from the remaining species of the genus. Novel molecular phylogenetic data of the new nematode were obtained, and were compared with those from available relatedProtospiruraspecies.AbstractA new spirurid nematode,Protospirura canariensisn. sp., a parasite of the black ratRattus rattusLinnaeus, 1758 (Rodentia: Muridae), in El Hierro Island (Canary Archipelago, Spain), was described by means of light (LM) and scanning electron microscopy (SEM). The most discriminating characteristics between the new species and the remaining species of the genusProtospirurawere observed to be the following: (a) the number of tooth-like outgrowths in the sub-median and lateral lobes of the pseudolabia, both in males and females (2 and 4, respectively); (b) the size of the right and left spicules in males (643–715 µm and 309–412 µm, respectively); and (c) the numbers and arrangements of the cloacal papillae in males. The new species has a total of 17 cloacal papillae (4 large and pedunculated pairs of precloacal papillae, an unpaired precloacal papilla and 4 pairs of postcloacal papillae). The arrangement of the postcloacal papillae are as follows: the first pair are large, pedunculated and placed near the posterior edge of cloaca; the three remaining postcloacal pairs are grouped and located near the posterior tip. In the latter group, the papillae in the first pair are large and pedunculated. The parasitized hosts and their geographical distributions were also useful criteria in distinguishingP. canariensisn. sp. from the remaining species of the genusProtospirura. In addition, the cytochrome c oxidase subunit I (cox1) sequence of the new species was obtained and compared with the available data of related species.