Species of Hysterothylacium Ward & Magath, 1917 (Nematoda: Anisakidae) have previously been reported only from marine and freshwater fishes. Here, we describe a new species that infects red-spotted newts (Notophthalmus viridescens), a North American amphibian species with fully aquatic adults. Aside from the unique characteristic of infecting an amphibian host, the new species differs from congeners by the presence of lateral alae, the length of intestinal cecum (0.54–0.73 mm, 39.67–49.09% of esophageal length), the size of the spicules (0.33–0.39 mm, 2.75–3.25% of body length), and the absence of tail tip ornamentation. The absence of fish in the ponds from which these specimens were obtained suggests that newts are the normal definitive host for this species. We suggest that this species might have diverged from a Hysterothylacium parasite of the freshwater fishes which usually live in close proximity with newts.

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