In view of their ecological importance and the abundance of threats on a developing Caribbean island, we surveyed the bats of Curaçao, Netherlands Antilles, and examined changes in the populations of seven threatened species over a decade, using previously published data as a baseline for comparison. The most important caves for bats (in terms of species representation and reproduction) were visited yearly, and monthly in 2001. Noctilio leporinus still occurs on the island, but does not appear to be numerous (six observed in 2003). We captured Myotis nesopolus nesopolus, but its roosting sites remain unknown. Leptonycteris curasoae curasoae numbers varied greatly, even within a year, and it may travel to and from other islands and Venezuela. Overall, however, the population of this species on Curaçao seems to be declining (1000 in 1993 and 625 in 2003); the disappearance of this pollinator could have severe consequences for the Curaçao ecosystem. Mormoops megalophylla intermedia is declining as well; in 2003, we counted 403 individuals including 75 pups, from 500 to 600 adults in the 10 previous years, representing a 25–30% decline in 1 year. We estimated the population of Natalus tumidirostris to be 890 in 2003. We also found a group of 60 Pteronotus davyi in Kueba di Ratón in 2003.Glossophaga longirostris elongata (1417 individuals counted) is the only species for which our data indicate relative stability over 10 years; L. curasoae and Mor. megalophylla are declining and other species must be monitored closely. Most caves are disturbed; four major caves require attention for the conservation of the most fragile species. Without immediate attention, Mor. megalophylla, in particular, risks imminent extinction. Despite problems associated with bat counts on Curacao, it is clear that regular surveys are crucial to understand bat populations and their fluctuations in caves, and to allow management responses to declines, particularly for areas undergoing rapid urbanization.