Northern Australia is one of few tropical places left on Earth in which biodiversity—and the ecological processes underpinning that biodiversity—is still relatively intact. However, scientific knowledge of that biodiversity is still in its infancy and the region remains a frontier for biological discovery. The diurnal Lepidoptera (butterflies and day-flying moths), and their intimate associations with vascular plants, exemplify these points. However, the opportunity to fill knowledge gaps is quickly closing: proposals for substantial development and exploitation of Australia’s north will inevitably repeat the ecological devastation that has occurred in temperate southern Australia—loss of species, loss of ecological communities, fragmentation of populations, and disruption of healthy ecosystem function—all of which will diminish the value of the natural heritage of the region before it is fully understood and appreciated. Patterns of species richness and endemic richness was analysed for 154 resident/breeding diurnal Lepidoptera in 153 grid cells (100 km x 100 km) based on arecently published set of spatial distribution maps (range-map and atlas data) in the western and central Australian Monsoon Tropics biome of northern Australia (~1.2 million km²).The north-western corner of the Top End – including the Arnhem Land Plateau (Kakadu NP-Nitmiluk NP and Warddeken IPA), reserves in the Darwin region, Litchfield NP, the Tiwi Islands, Fish River-Daly River, and Cobourg Peninsula (Garig Gunak Barlu NP) – was identified as a major biodiversity hotspot for the conservation of diurnal Lepidoptera based on congruent patterns of species richness, endemic richness and weighted endemism.