ABSTRACT Freshwater leeches were collected from Chiapas, Mexico. Among these collections and prior records, a total of nine species were found, comprising six genera and four families. One species, Diestecostoma octannulata, represents a new record for Mexico and two species, Helobdella elongata and H. octatestisaca, represent new records for Chiapas. Additionally, a new species of Erpobdella was discovered from a single stream in the park El Arcotete near San Cristobal de las Casas. Here it is described as Erpobdella adani and it is morphologically distinguished from Mexican Erpobdella species by the combination of having the whole body strongly dorsoventrally flattened, three annuli between gonopores, an enlarged male gonopore, and no preatrial loops for the male reproductive system. Based on sequence data (mitochondrial cytochrome c oxidase subunit I and 12S; nuclear 18S), this new species appears most closely related to a clade formed by E. coastalis and E. ochoterenai. Both mitochondrial cytochrome c...