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Pulmonary actinomycosis is described in 17 South Australian koalas necropsied between 2016 and 2019. From these cases, four koalas had secondary hypertrophic osteopathy. Plain radiographical and computed tomography images demonstrated periosteal reaction on multiple appendicular skeletal bones in all cases, including scapula, humerus, ulna, radius, ilium, femur, tibia, fibula, metacarpus, metatarsus and phalanx. Grossly, periosteal surfaces of the metaphyses and diaphyses of long bones were thickened and roughened; microscopically, this was characterised by bi-layered proliferation of well-differentiated trabecular bony spicules oriented perpendicular to the cortex (pseudocortices) and separated by vascular connective tissue, typical for hypertrophic osteopathy. Well characterised in domestic species and rarely reported in marsupials, this is the first radiographical and pathological characterisation of hypertrophic osteopathy in koalas, associated with pulmonary actinomycosis in all cases., (© 2021 Australian Veterinary Association.)