Xeronycteris vieirai is an endemic bat species described by Gregorin and Ditchfield in 2005 only five specimens of which have been collected to date. It is considered Data Deficient by the IUCN Red List. There is little information about this species, so its ecological requirements are poorly known although its diet has been speculated to be nectarivorous. Xeronycteris vieirai apparently has a distribution restricted to the semi-arid Brazilian Caatinga, a habitat that has been highly degraded. We present here the first information about the natural history of X. vieirai, obtained through a large research project aiming at the ecology and conservation of bats in the Caatinga of the states of Rio Grande do Norte and Pernambuco, northeastern Brazil. From June 2014 to October 2015 we captured 62 individuals of X. vieirai at five sites, which represent new records and expand its known distribution, establishing the northernmost record. Xeronycteris vieirai feeds on nectar of at least nine species of plants belonging to six families, and mainly on the columnar cactus Pilosocereus pachycladus. The species has peak activity at 1830 h and shows an apparently seasonally monoestrous reproductive cycle (June–December). Adult females have longer forearms than adult males and are significantly heavier; there is no bias in the sex ratio. A colony of approximately 20 individuals was observed in a well-ventilated cave with natural lighting, representing the first documented roost for the species. Finally, ectoparasites of the genus Trichobius were found on 85.7% of the individuals examined, with no difference in parasite load between males and females.