The new genus Isalomyia is erected for a new species, I. irwini, discovered in the Isalo Range of south-western Madagascar, the first vermileonid known from that island. Morphological adaptations of the proboscis for nectarivory show this taxon to be part of the Gondwanan monophylum of genera. Only the female of I. irwini is known, so a full account of Isalomyia and an assessment of its relationships cannot be given yet. The antennal form is plesiomorphic, but the remarkable way in which sensilla on the flagellomeres are distributed is distinctive and apomorphic. The wing venation shares features with the Omani species Lampromyia umbraticola Stuckenberg & Fisher, 1999, but these may be plesiomorphies. I. irwini was found in a topographically unique area with sandstone inselbergs and outcrops on the eastern margin of the Permo-Triassic sediments in western Madagascar. Landforms produced by erosion-resistant sandstone formations may provide durable larval habitats for vermileonid faunas. Such landforms are significant for vermileonid biogeography in South Africa, and could prove to be important in Madagascar.