Female nematodes were collected from the intestines of a road-killed Yucatan Neotropical rattlesnake, Crotalus tzabcan, in Yucatan, Mexico. Morphometric analysis of the nematode specimens allowed their identification to the genus Kalicephalus. However, the absence of male specimens prevented species-level identification. Notably, second- and third-stage larvae were observed in the reproductive tract of some Kalicephalus females. Additionally, the internal transcribed spacer (ITS) region, the cytochrome c oxidase subunit 1 (COI) gene, and the 28S rRNA gene (28S) were amplified and sequenced. Pairwise genetic distances between the Kalicephalus sp. sequences from C. tzabcan and those of other Kalicephalus species were calculated for the three molecular markers. In the phylogenetic tree (ITS1) of Ancylostomatina, the sequences of Kalicephalus sp. were clustered with Kalicephalus viperae from Iran with high support. This study reports, for the first time, the genus Kalicephalus in C. tzabcan, and the presence of larvae in the reproductive tract of Kalicephalus females naturally parasitising a snake.
