A new species of Serpentirhabdias Tkach, Kuzmin & Snyder, 2014 (Nematoda: Rhabdiasidae) parasitic in the brown ground snake Atractus major Boulenger (Reptilia: Serpentes: Dipsadidae) in Brazil.

Serpentirhabdias atracti n. sp. is described based on specimens discovered in the lung of Atractus major Boulenger from Caxiuanã National Forest, Pará, Brazil. The new species is assigned to Serpentirhabdias Tkach, Kuzmin & Snyder, 2014 based on morphological characters (comparatively thin body cuticle without prominent inflations, arrangement of circumoral papillae in two lateral groups, pre-equatorial position of vulva, eggs in uteri at early cleavage stages), as well as because of its parasitism in snakes. The new species is most similar to S. vellardi (Pereira, 1928) due to the absence of lips and buccal capsule, similar body dimensions, and the specificity to dipsadid snakes in Brazil. The two species differ in the shape of the tail (bulbous dilatation in the posterior part followed by a thread-like tail tip present in S. atracti n. sp.), the width of the oesophagus, and the size of the excretory glands. Serpentirhabdias atracti n. sp. is the sixth species of this genus found in the Neotropical Region.