SUMMARY
Bat communities of mainland Southeast Asia can be highly diverse. Many are under threat. Despite this, regional faunal composition is not well documented for many areas, including regions of Vietnam. We assessed the biodiversity of bats in a watershed protection forest in the southeastern Truong Son (Annamite) Mountains, southwestern Quang Ngai Province, Vietnam in 2011–2013. Twenty species of insectivorous bats were documented including a high diversity of Murina species Tube-nosed Bats. Diversity and abundance indices were compared with that recorded previously in two nature reserves and one national park in Vietnam, and were higher or comparable in several measures despite the lack of a karst substrate for roosts. Reproduction in the insectivorous bat fauna coincided with the early rainy season. In the late dry season, pregnant females of several species were observed but volant juveniles were not present, whereas in the early wet season adult females were lactating or post-lactating and volant juveniles of nine species were detected. We recorded echolocation calls of 14 bat species; for each species, we compared features of calls with those reported previously in other Asian localities. For some species we found discrepancies in call metrics among studies, perhaps suggesting a greater hidden biodiversity of bats in Southeast Asia.