The mean liver concentrations of copper, manganese, zinc and cobalt were 0.25, 0.20, 2.97 mmol/kg and 2.81 mumol/kg respectively in free-living koalas in Victoria, Australia. The mean plasma copper concentration was 9.2 mmol/liter which was somewhat below the level in other hindgut fermenters. The mean concentrations of copper, manganese and zinc in their diet (Eucalyptus spp.) were 0.08, 4.46 and 0.27 mmol/kg respectively. Analysis of the data established a significant correlation between the age of the koalas and the inverse of the concentration of the copper (r = -0.67, P less than 0.001) in the liver. There were no such correlations apparent for manganese, zinc or cobalt. The concentrations of trace metals in the Eucalyptus diet for the koala were comparable to those recommended in the diets for other hindgut fermenters such as horse, rabbits and rats. However there was evidence for suboptimal plasma copper levels (mean 9.2 mmol/liter) in some koalas, and reduced liver copper levels in older koalas. Liver histology revealed the presence of brown intracytoplasmic granules in hepatocytes. The size and number of these granules per cell was noted to increase with increasing age of the koala but the chemical nature and the role of the granules was not determined by the histochemical techniques used.