Investigations of the amphibians and their environments in the Isthmus of Tehuantepec have been presented with the aim of gaining an understanding of the present biological and of the historical events responsible for the present patterns of distribution of amphibians in this region. The Isthmus of Tehuantepec embraces three major environments--savanna, semi-arid scrub forest, and quasi-rainforest. The rainforest presents an environment noticeably different from the other two and has a different amphibian fauna. Analysis of present patterns of distribution shows that certain species are restricted to the rainforests on the Gulf lowlands; others live only in the semi-arid scrub forests on the Pacific lowlands. A third group of species lives on both the Gulf and Pacific lowlands; most of these species occur only in the scrub forests or savannas on the Gulf lowlands, but some also inhabit the rainforest. In one way or another the isthmus presents a barrier to the distribution of 75 per cent of the species of amphibians living in the lowlands; it is a greater barrier still to the species inhabiting the highlands on either side. Present patterns of distribution are attributed to bioclimatic fluctuation in the Pleistocene. In the course of these climatic shifts, tropical environments and their amphibian inhabitants seem to have survived in the isthmian region. The amphibian fauna of the lowlands of the Isthmus of Tehuantepec consists of 16 genera and 36 species. Systematic studies of all available specimens from the region show that _Eleutherodactylus conspicuus_ Taylor and Smith is a synonym of _Eleutherodactylus alfredi_ Boulenger and that _Hyla axillamembrana_ Shannon and Werler is a synonym of _Hyla loquax_ Gaige and Stuart. LITERATURE CITED BEARD, J. S. 1953. The savanna vegetation of northern tropical America. Ecol. Mono., vol. 23 (2):149-215. BOULENGER, G. A. 1898. Fourth report on additions to the batrachian collection in the Natural History Museum. Proc. Zool. Soc. London, 1898, pp. 473-482, pls. 38-39. BURT, C. E. 1931. A study of the teiid lizards of the genus _Cnemidophorus_ with special reference to their phylogenetic relationships. Bull. U. S. Nat. Mus., No. 154, pp. viii + 286. CONTRERAS A., A. 1942. Mapa de las provincias climatologias de la Republica Mexicana. Mexico, D. F., Direccion de Geografia, Meterologia e Hidrologia, pp. i-xxviii, tables 1-54, 2 maps. COOKE, C. W. 1945. Geology of Florida. Florida Geol. Surv., Geol. Bull., No. 29:1-339. DORF, E. 1959. Climatic changes of the past and present. Contrib. Mus. Paleo. Univ. Michigan, vol. 13 (8):181-210. DUELLMAN, W. E. 1956. The frogs of the hylid genus _Phrynohyas_ Fitzinger, 1843. Miscl. Publ. Mus. Zool. Univ. Michigan, No. 96:1-47, pls. 1-6.