Our knowledge of the stygobitic (aquatic, cave-limited) fauna from the Yucatan Peninsula, Mexico is still in an early stage. All previously published taxonomic descriptions of the fauna have been based strictly on morphological characteristics. However, fundamental questions pertaining to systematics, biogeography, evolutionary origins and population dynamics can be best addressed with modern genetic techniques such as DNA sequencing of nuclear and mitochondrial genes. The purpose of this study is to determine the intraspecific population structure for the shrimp Typhlatya mitchelli (Fig. 1) found in the anchialine caves of Quintana Roo and Yucatan, Mexico by sequencing fragments of multiple genes. This widespread stygobitic genus contains 10 described species from fresh and salt water caves in the Atlantic, Mediterranean and western Pacific (Fig. 2). Mitochondrial genes including cytochrome b, cytochrome oxidase I, and 16S rRNA were sequenced (Fig. 3), since they have proven useful in other crustacean population studies (Palumbi & Benzie, 1991; Knowlton et al., 1993; Machado et al., 1993). Genomic DNA provided the starting template for all PCR reactions. Shrimp specimens from multiple cave systems across the Yucatan Peninsula (Fig. 4) were collected and genetic variability within and among these populations was assessed to test the effects of geographic distribution on genetic relatedness. Typhlatya pearsei was used as an outlier group because it too inhabits caves and cenotes throughout the Yucatan Peninsula and is believed to have colonized caves at about the same time as T. mitchelli (Wilkens, 1982). All three genes sequenced show very little divergence, either within population or across the entire study area. These results lead to several possibilities: 1) the selected genes do not change over time, 2) these shrimp recently colonized these caves and have not had sufficient time to diverge genetically, or 3) there is gene flow across the Yucatan through an extensive network of underground cave systems. The large number of published studies on these mitochondrial genes and their frequent use in population studies should rule out the first possibility. An unusually high number of relict taxa are found in anchialine caves worldwide (Iliffe, 2000), including the Yucatan, suggesting that these caves have been inhabited for a long period of time and therefore ruling out the second option. Possibility number three seems to be the best explanation for the data and the sequencing of mitochondrial genes from additional stygobitic taxa (e.g., amphipods, mysids, isopods, copepods or other shrimps) could strengthen this hypothesis.