Caliciopsis pleomorpha sp. nov. is described from a severe stem canker disease of cultivated Eucalyptus cladocalyx 'Nana' (dwarf sugar gum) in Australia. The fungus is a pleomorphic ascomycete (Coryneliales), with pycnidial (pleurophomalike) and hyphomycetous (phaeoacremonium-like) morphs, and differs in these respects and in ITS sequences from other Caliciopsis spp. The fungus was also found associated with cankers on other Eucalyptus species growing in native habitats, and was successfully inoculated under glasshouse conditions into a wide range of Eucalyptus species on which it caused cankers of varying severity.