In analyzing the distribution of the amphibians with respect to those that are restricted to either the Pacific or Gulf lowlands or those that cross the continental divide in the isthmus, we find that 25 per cent of the species are restricted to the Gulf lowlands, 17 per cent are restricted to the Pacific lowlands, and 53 per cent cross the isthmus. In analyzing the distribution patterns with respect to those that extend across the isthmus of Tehuantepec from east to west, we find that 14 per cent of the species do not extend east of the isthmus into Central America and that 19 per cent do not range west of the isthmus into Mexico proper; 61 per cent of the species range to the east and to the west of the isthmus. Of the 36 species of amphibians inhabiting the isthmus only nine species (25 per cent) range across the isthmus, that is, occur on the Gulf and Pacific lowlands, and also range to the east and to the west of the isthmus. To these wide-ranging species the diversified environments of the isthmus do not present a barrier to distribution. The other 27 species (75 per cent) either do not cross the isthmus from east to west or from north to south; thus, probably in one way or another the isthmus presents a barrier to their distribution. THE AMPHIBIAN FAUNA OF THE FOOTHILLS AND ADJACENT HIGHLANDS To amphibians inhabiting the foothills and mountains of southern Mexico and northern Central America, the isthmus presents a great barrier to dispersal. For example, salamanders of the genus _Thorius_, the _mexicanus_ and _augusti_ groups of the genus _Eleutherodactylus_, the _bistincta_ group of the genus _Hyla_, and the genus _Tomodactylus_ occur on the Mexican Plateau and southward into the mountains of Oaxaca. Nevertheless, no members of these groups are present in the Guatemalan-Chiapan Highlands. The genera _Chiropterotriton_, _Magnadigita_, _Pseudoeurycea_, and _Ptychohyla_, as well as the _eximia_ group of _Hyla_ are represented by different species in the Guatemalan-Chiapan Highlands than in the mountains of Mexico on the other side of the isthmus. Several species of _Plectrohyla_ occur in the Guatemalan-Chiapan Highlands, but none is known from the Mexican Highlands, although one species occurs in the Tuxtlas.