The bumble-bee bat Craseonycteris thonglongyai of the monospecific family Craseonycteridae is categorized as Endangered on the IUCN Red List. First discovered in 1973, it was until recently only known from a small population of approximately 2,300 individuals restricted to the catchment area of the River Kwai, Thailand. However, in 2001 a single craseonycterid was discovered in Mon State, Myanmar, extending its geographical range by approximately 250 km. In October and November 2002 a survey was undertaken to examine the status of C. thonglongyai in Myanmar and assess its geographical distribution and population size. C. thonglongyai calls were recorded from bats emerging from nine of 19 caves surveyed; the population size was estimated to be 1,500. The phylogenetic relationships between the Thai and Myanmar populations were investigated using molecular, morphological and acoustic data. Morphologically, the two populations are indistinguishable. However, there is an 8–10 kHz echolocation call divergence between the populations. Cytochrome b data suggest that the two populations are similar and that the Myanmar population may be monophyletic. Annual surveys of the known bat roosts and in situ education programmes for local people are recommended together with the establishment of an integrated, transboundary Myanmar/Thai conservation strategy.