OBJECTIVE To diagnose and characterise thymic lymphosarcoma in a koala. METHODS A pathological case. METHODS Seven-year-old female koala. METHODS The neoplastic process was investigated macroscopically, haematologically, histologically and immunohistologically. RESULTS The koala had difficulty swallowing because of a medial swelling in the lower neck. Biopsy of this mass and blood examination revealed lymphosarcoma with a leukaemic manifestation; necropsy and histopathological examination showed the mass to be thymus. Palatine tonsils, cervical, axillary and mesenteric lymph nodes, spleen, liver, gut, bronchi, genitalia and bone marrow were infiltrated by neoplastic cells. Immunohistological staining of the thymic mass, cervical and mesenteric lymph nodes, bone marrow, spleen and gut revealed the neoplastic cells to be of T lymphocyte origin (positive for both anti-human CD3 and CD5). CONCLUSIONS It is speculated that the neoplastic process originated in the thymus and was disseminated by bloodborne neoplastic cells. This first report of thymic lymphosarcoma in a marsupial confirms that antibodies raised originally to investigate human lymphoid neoplasia can cross-react with neoplastic lymphocytes in koalas.