Eucalyptus regnans, known variously as mountain ash (in Victoria), giant ash or swamp gum (in Tasmania), or stringy gum, is a species of very tall forest tree that is native to the Australian states of Tasmania and Victoria. It is a straight-trunked tree with smooth grey bark, but with a stocking of rough brown bark at the base, glossy green, lance-shaped to curved adult leaves, flower buds in groups of between nine and fifteen, white flowers, and cup-shaped or conical fruit. It is the tallest of all flowering plants; the tallest measured living specimen, named Centurion, stands 100 metres (328 feet) tall in Tasmania.
It often grows in pure stands in tall wet forest, sometimes with rainforest understorey, and in temperate, high rainfall areas with deep loam soils. A large number of the trees have been logged, including some of the tallest known. This species of eucalypt does not possess a lignotuber and is often killed by bushfire, regenerating from seed. Mature forests dominated by E. regnans have been found to store more carbon than any other forest known. The species is grown in plantations in Australia and in other countries. Along with E. obliqua and E. delegatensis it is known in the timber industry as Tasmanian oak.
