jats:pMortality factors are an important subject of research, especially when mortalities concern threatened taxa. The grey-headed flying-fox (Pteropus poliocephalus) is a vulnerable species endemic to eastern and south-eastern Australia that is known to be susceptible to a spectrum of compounding threats including factors producing simultaneous deaths. We describe two incidents of trees within a flying-fox roost collapsing and causing mass mortalities in individuals roosting in those trees. Each incident resulted in 335 and 57 deaths, as well as 74 and 45 individuals that would have died if not taken into ex-situ care. Total mortality for each incident equated to approximately 15% and 2% of the colony at the time of those incidents. Although flying-foxes are capable of flight, take-off from a roosting position requires an initial freefall, which makes escape from a collapsing tree difficult. Our accounts potentially represent the first reports of tree collapses causing mortality in Australian flying-foxes, which highlight the relevance of tree health in roosts to flying-fox welfare and conservation. This information also builds upon our growing knowledge of factors producing mass mortalities in flying-foxes.</jats:p>