Syzygium neruosum is a common monsoon rain forest tree. Its habitat in Australia consists of small rain forest patches that are scattered through a savanna matrix. It is a mast flowering canopy species that produces large quantities of fruits fed on by mobile frugivores such as birds and fruit bats. The genetic diversity of this species was investigated, especially in relation to rain forest patch size, geographic isolation, and geographic distribution. Syzygium neruosum was found to have high levels of genetic diversity within populations (He = 0.307). Diversity among populations, however, was relatively low (Fsr = 0.118), and was nor spatially structured across its geographic range in Australia. ’This is thought to have been caused by relatively frequent gene flow among populations (N,,, = 1.67), mediated primarily by mobile frugivores. Genetic diversity was not correlated with patch size or isolation. It is thought that seed dispersal by frugivores has acted to expand the effective population size of this species beyond the individual rain forest patch, and thus has prevented the substantial loss of genetic diversity that otherwise would have been observed. Thus this species is dependent upon these frugivores for the maintenance of its genetic diversity and hence its longterm viability. These results lend support to theories of post-Holocene expansion of rain forest by vagile species in northern Australia.