This study analyzes the ecological distributions of reptiles and amphibians in southern Tamaulipas of northeastern Mexico. Observations are confined to a small, though topographically complex, section of the Sierra Madre Oriental to enable a more careful definition of zonal distribution than would be possible had the same amount of fieldwork been expended in a larger geographical unit. Geology, climate, and vegetation are environmental features of primary concern to the animal ecologist, and this study discusses each of these in turn. Such information should clarify the environmental basis for certain distribution patterns both throughout eastern Mexico and, locally, in the Gomez Farias region. In addition it should be useful in comparing this with other peritropical areas.