Molecular data reveal a new species of Stiles & Hassall, 1898 (Digenea, Echinostomatidae) in the Common opossum, L. (Mammalia, Didelphidae) in the Yucatán Peninsula, Mexico.

A new species of Stiles & Hassall, 1898 is described from the small intestine of the Common opossum, Linnaeus from the Yucatán Peninsula, Mexico. is morphologically very similar to the type species of the genus, (Rudolphi, 1819) Stiles & Hassall 1898, a species widely distributed in opossums across Mexico. A molecular phylogenetic analysis using a mitochondrial gene (cox1), and the nuclear ribosomal internal transcribed spacer region (ITS1-5.8S-ITS2), of specimens of collected in several localities of Mexico revealed that those from the Yucatán Peninsula, originally recorded on morphological grounds as actually represented an independent genetic lineage. Maximum Likelihood and Bayesian Inference analyses were performed for each data set independently, and for the concatenated data set (ITS1-5.8S-ITS2 + cox1). All phylogenetic analyses showed that the specimens from Yucatán represented a monophyletic lineage, with high bootstrap support and Bayesian posterior probabilities. In addition, the genetic divergence estimated between and two species of , , and Chandler, 1932 that also occur in Mexican marsupials ranged between 7-8% and 16-17%, for cox1, and between 0.1-0.2% and 7% for the ITS region, respectively. The molecular evidence gathered in this study (reciprocal monophyly in both phylogenetic analyses, and estimated genetic divergence) suggested that the specimens found in the intestine of originally reported as from Yucatán, actually represent a new species. Morphological evidence was found through light and scanning electron microscopy to support the species distinction based on molecular data.