A result of extensive field work in southern Veracruz, Mexico during 1960 and 1962, this pioneering biological work is now in print for the first time. The Sierra de Tuxtla, a small isolated volcanic mountain area near the Gulf coast of southern Veracruz, Mexico, affords excellent conditions for biogeographical investigations. This study describes the manner in which physical and human factors have influenced native vegetation, nontransient avifauna and larger forest mammals in this tropical region. Physical factors considered include geology, land configuration, climate, drainage and soils; human factors examined include population, settlement and forms of land use.