The Kimberley region of Western Australia is recognised for its high biodiversity and many endemic species, including the charismatic boab tree, Adansonia gregorii F.Muell. (Malvaceae: Bombacoideae). In order to assess the effects of biogeographic barriers on A. gregorii, we examined the genetic diversity and population structure of the tree species across its range in the Kimberley and adjacent areas to the east. Genetic variation at six microsatellite loci in 220 individuals from the entire species range was examined. Five weakly divergent populations, separated by westâeuro"east and coastâeuro"inland divides, were distinguished using spatial principal components analysis. However, the predominant pattern was low geographic structure and high gene flow. Coalescent analysis detected a population bottleneck and significant gene flow across these inferred biogeographic divides. Climate cycles and coastline changes following the last glacial maximum are implicated in decreases in ancient A. gregorii population size. Of all the potential gene flow vectors, various macropod species and humans are the most likely.