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The aquatic fauna of Mexico and Central Americaincludes cavernicole species derived from both freshwaterand marine ancestors; the terrestrial faunaincludes species with both temperate and tropicalaffinities. The purpose of the present discussion is tooutline briefly the distribution of some speciesbelonging to each category.Aquatic TroglobitesMarine relicts.-Species which are apparentlyderived from marine ancestors include triclad turbellarians,isopods, amphipods, mysids, shrimp, andfish.The flatworm family Dimarcusidae is known onlyfrom Mexican caves and is apparently a member ofthe marine suborder Maricola and without obviousclose relatives. Two species of this family, Ophistobursamexicana Benazzi and O. josephinae Benazzi,occur in caves in Tabasco and Chiapas respectively.Both have been found in association with bogidiellidamphipods, another group of marine relicts.The isopod fauna includes representatives of thefamilies Cirolanidae, Stenasellidae, Anthuridae, andMicrocerberidae. The distribution of the Cirolanidaeis of particular interest. Most of the species areknown from one or more caves in the same karstregion. This is true of Speocirolana thermydronisCole and Minckley, Mexilana saluposi Bowman, andSphaerolana interstitialis Cole and Minckley. Speocirolanabolivari (Rioja) has been found only in theSierra de El Abra, Sierra de Guatemala, and Aquismonregions, contiguous karst regions along the easternface of the Sierra Madre Oriental in San LuisPotosi and Tamaulipas. Sphaerolana affinis Cole andMinckley occurs both in the Cuatro Cienegas deCarranza region, Coahuila, and Sierra de la Silla,Nuevo Leon. Creaseriella anops (Creaser) has beenfound both in the Coastal Plain and Sierra de Ticul ofthe Yucatan Peninsula. This is a continuous limestonearea, and subterranean dispersal is likely. Of someinterest is the distribution of Speocirolana pelaezi(Bolivar). This species is the most widely distributedaquatic troglobite in Mexico and has been found inthe Sierra de Tamaulipas and Sierra de Guatemala,Tamaulipas; the Sierra El Pino, Sierra de El Abra, andAquismon regions, San Luis Potosi; and the Cuetza­Ian region, Puebla. It is likely, therefore, that itsrange extends the length of the Sierra Madre Oriental.It has an altitudinal range of about 1200 meters.The stenasellid isopods of Mexico range fromCoahuila to Veracruz. The genus Mexistenasellus isrepresented by species in the Cuatro Cienegas deCarranza, Micas, and Atoyac regions. Each species isrestricted to subterranean habitats in its respectiveregion. The monotypic genus Etlastenasellus hasbeen collected only from a well in Oaxaca and ispresumably a phreatobite.The family Anthuridae contains two Mexicantroglobites, one each in a cave in the Atoyac regionof Veracruz and in the Micos region of San LuisPotosi.The only described microcerberid isopod knownfrom Mexico is Mexicerberus troglodytes Schultz,which appears to be endemic to the Sierra de Guatemala,Tamaulipas. Other species of this family occuras inhabitants of the interstitial zone in Cuba.Two families of amphipod, both apparently ofmarine origin, have contributed to the cavernicolefauna of Mexico and Guatemala. The Bogidiellidae,with the single genus Bogidiella, is represented bynine species in extreme southern Mexico (Oaxaca,Chiapas, Veracruz, Tabasco) and Guatemala (Huehuetenango,Alta Verapaz). Possibly undescribed specieshave been taken in southern Puebla and Campeche.Several of the species are known only from wells andmay be phreatobites, but others are clearly caveassociated. Two genera of the family Hadziidae havebeen described from Mexican caves (Mayaweckeliawith two species in Yucatan and adjacent Campecheand Mexiweckelia with two species in the CualroCienegas de Carranza region, Coahuila, and one speciesin the Rancho Descubridora region, Durango.The order Mysidacea includes four Mexican troglobitesin two families. The Lepidomysidae is representedby Spelaeomysis olivae Bowman from a cavein the Acatlan region of Oaxaca and by S. quinterensis(Villalobos) from caves in the Sierra de El Abraand Sierra de Guatemala. Two species of the familyMysidae are troglobites in Mexico: Antromysis (A.)cenotensis Creaser from the Coastal Plain of theYucatan Peninsula and A. (A.) reddelli Bowman froma cave in the Acatlan region of Oaxaca.Most of the troglobitic shrimp inhabiting the cavesof Mexico are probably derived from marine ancestors.This speculation is greatly strengthened by thefact that they are found only in the areas borderingthe Gulf of Mexico and Caribbean Sea. Three familieshave been found in Mexican caves; the Alpheidae,Atyidae, and Palaemonidae.The Alpheidae is represented only by Alpheopsisstygicola Hobbs from caves in the Acatlan region ofOaxaca. With the exception of two Old World species,all members of this family are marine.The Atyidae includes three species of the genusTyphlatya in the Yucatan Peninsula. Although thefamily Atyidae includes both freshwater and marinespecies, the distribution of the genus Typhlatya(Cuba, Puerto Rico, Dominican Republic, Ascension60