In all, 28 koalas (Phascolarctos cinereus), 26 of which had died exclusively of natural causes, were autopsied and in 21 of these a probable cause of death could be recognized. Different forms of pneumonia head the list of these causes with six cases, including two where the primary lesion was trauma. Hepatitis with suppurative cholangitis was observed in three instances. Cryptococcosis, an infection by the yeast Cryptococcus neoformans, was responsible for three deaths, and two forms of blood dyscrasia, i.e. lymphoblastic leukaemia, and an anaemia of unknown origin accounted for two more deaths. Cystic disease of the ovary was observed in six koalas, and in four cases was complicated by infection and was the main cause of death. Middle ear sepsis, ulcerative colitis, and cardiac failure associated with senility were seen once each. In the remaining seven cases the cause of death was indeterminate, though senility appeared to be the predisposing cause in two.