1. SPECIES RESTRICTED TO THE GULF LOWLANDS. Of the 36 species of amphibians recorded from the Isthmus of Tehuantepec, nine (25 per cent) are in this group. Four of these (_Eleutherodactylus alfredi_, _Syrrhophus leprus_, _Hyla loquax_, and _Hyla picta_) live in the Gulf lowlands to the east and to the west of the isthmus. Three others (_Hyla ebraccata_, _Hyla microcephala martini_ and _Phyllomedusa callidryas taylori_) are primarily Central American in their distribution and reach the northwestern limits of their ranges in the Gulf lowlands of the isthmus, whereas _Bolitoglossa platydactyla_ and _Eleutherodactylus natator_ reach the southern limits of their distributions in the isthmus. 2. SPECIES RESTRICTED TO THE PACIFIC LOWLANDS. This group includes six species, or 17 per cent of the amphibian fauna of the isthmus. Two of these (_Bufo coccifer_ and _Syrrhophus pipilans_) range to the east and to the west of the isthmus on the Pacific lowlands. Two others (_Bufo canaliferus_ and _Hyla robertmertensi_) range from the isthmus into Central America, and _Diaglena reticulata_ and _Phyllomedusa dacnicolor_ range on the Pacific lowlands of Mexico southeastward to the isthmus. 3. SPECIES THAT OCCUR ON THE PACIFIC AND GULF LOWLANDS. This group includes 19 species, or 53 per cent of the total amphibian fauna. Of these, nine species (25 per cent of the entire amphibian fauna) are widespread throughout the lowlands of Mexico and Central America; these are: _Gymnopis mexicanus_ _Rhinophrynus dorsalis_ _Bufo marinus_ _Engystomops pustulosus_ _Leptodactylus labialis_ _Leptodactylus melanonotus_ _Hyla baudini_ _Hyla staufferi_ _Rana pipiens_ Four species occur on the Gulf lowlands to the east and to the west of the isthmus, but on the Pacific lowlands they occur only to the east; this group includes _Bufo valliceps_, _Eleutherodactylus rhodopis_, _Phrynohyas modesta_, and _Phrynohyas spilomma_. Three species live to the east and to the west of the isthmus on the Pacific lowlands, but only to the west on the Gulf lowlands; these include _Eleutherodactylus rugulosus_, _Microbatrachylus pygmaeus_, and _Gastrophryne usta_. Six species that cross the isthmus live on the humid Gulf lowlands and on the humid lowlands of Chiapas and Guatemala, but not on the semi-arid Plains of Tehuantepec; these include _Bolitoglossa occidentalis_, _Eleutherodactylus rhodopis_, _Microbatrachylus pygmaeus_, _Phrynohyas modesta_, _Phrynohyas spilomma_, and _Rana palmipes_. Of these, _Microbatrachylus pygmaeus_ also occurs in scattered humid environments to the west of the isthmus on the Pacific lowlands. Two species are endemic to the isthmian region. _Bolitoglossa veracrucis_ is known only from the humid northern slopes of the isthmus. _Hylella sumichrasti_ occurs on the Pacific slopes of the isthmus and extends to the east into western Chiapas.