On the basis of molecular and morphological evidence, Mormopterus acetabulosus, hitherto considered an endemic to the Mascarene Islands (Mauritius and La Réunion), is shown to comprise 2 closely related taxa. The holotype of M. acetabulosus is from Mauritius and the new taxon described herein is from La Réunion. M. acetabulosus from Mauritius is notably larger than members of this genus from La Réunion, and several soft-part and cranial characters distinguish these 2 taxa. This conclusion is supported by examination of mitochondrial DNA control region data for 141 bats, which shows these 2 groups to be reciprocally monophyletic, separated by an average of 5.01% uncorrected sequence divergence. Two nuclear intron regions (7th intron of the beta fibrinogen gene and thyrotropin) also were included, but showed limited genetic variation and no fixed differences between the 2 taxa. These 2 species of Mormopterus are common on Mauritius and La Réunion, often living in caves or synanthropically, and are not considered a conservation concern.