AbstractThe first molecular phylogeny for mountain reed snakes (genus Macrocalamus) based on the mitochondrial gene cytochrome b is not entirely consistent with the previous taxonomy based on morphology and colour pattern. Macrocalamus chanardi is shown to be a species complex composed of three different allopatric lineages distributed across different upland areas in Peninsular Malaysia that are morphologically conserved but genetically distinct. A new and morphologically different species, Macrocalamus emas sp. nov., is described from the Cameron Highlands, Pahang, Peninsular Malaysia. It occurs in sympatry with four other ecologically equivalent species of Macrocalamus and one other species of Collorhabdium. The phylogeographical pattern of sympatric genetically distinct species of Macrocalamus endemic to upland areas is attributed to the fossorial nature of these snakes and the montane forest expansion and retraction resulting from cyclical, glacioeustatically driven climatic processes that have reconstructed the geography of Sundaland continuously over the last 25 Myr.