Caves are very important roosts for bats. Neotropical countries, like Brazil, harbor both high bat species richness and abundance of underground cavities. The Caatinga drylands in northeastern Brazil are a good example with potentially 100 bat species and a high number of caves. Among those caves, some harbor exceptionally large bat populations, but, even thus, they still remain poorly known and studied. Here we present a 48-months monitoring based on a non-invasive thermal detection system to estimate the abundance changes of a large colony in a bat cave in the Caatinga drylands of Brazil. That cave harbors ten bat species — two nationally endangered — and 70% of them are main cave users. We found a marked inter-month fluctuation in the total bat abundance, with no consistent pattern. Total bat abundance was neither related to precipitation nor to the reproductive seasonality. During certain moments, the bat population can reach up to 120,000 individuals, making this cave a priority site for bat conservation in Brazil.