The amount of differentiation in isolated populations of amphibians in southern Mexico and northern Central America gives some idea of relative lengths of time of isolation from related populations. Those populations inhabiting high mountain environments on either side of the isthmus are specifically distinct. Some populations inhabiting cloud forests lower on the mountains are specifically distinct from related populations on the other side of the isthmus; between others there is no recognizable differentiation. Even though many populations are isolated from other populations of the same species in the lowlands of the isthmus, there is no apparent speciation. This indicates that the lowland environments and their inhabitants have been isolated from one another for a shorter time than have the highland environments and their inhabitants. ACCOUNTS OF SPECIES For each species of amphibian known to occur in the lowlands of the Isthmus of Tehuantepec, localities where one or more specimens were collected are listed, and variation, ecology, and life histories are discussed. A total of 2833 specimens has been examined for the purposes of this study. Individual specimens cited in the text are listed with catalogue numbers and abbreviations of the name of the museum, as follows: AMNH American Museum of Natural History KU University of Kansas Museum of Natural History MCZ Museum of Comparative Zoology, Harvard College UIMNH University of Illinois Museum of Natural History UMMZ University of Michigan Museum of Zoology USNM United States National Museum =Gymnopis mexicanus mexicanus= Dumeril and Bibron _Oaxaca_: El Barrio (3); Matias Romero; Tehuantepec (2). _Veracruz_: Cosamaloapan; Cuatotolapam (2). The two specimens from Cuatotolapam were collected by Ruthven in an area of mixed savanna and forest. The three specimens (USNM 30535-7) listed above from El Barrio were collected by Sumichrast; possibly they came from another locality. The city of Tehuantepec is divided into seven districts called "barrios." The two specimens listed from Tehuantepec (MCZ 1604) merely bear the data "Tehuantepec, Mexico." They may have come from the town, the district, or from anywhere in the isthmus. The specimen from Matias Romero has 109 primary and 67 secondary annuli, a length of 400 mm., and a diameter of 19 mm.; the one from Cosamaloapan has 106 primary and 58 secondary annuli, a length of 397 mm., and a diameter of 19 mm. Data on the other specimens were recorded by Dunn (1942:475). =Bolitoglossa occidentalis= Taylor _Oaxaca_: Rio Sarabia (2); Ubero. _Veracruz_: La Oaxaquena; 14 km. E of Suchil.