This paper reports the occurrence of a spongiform encephalopathy (SE) in young adult monkeys housed in a zoological park. Three rhesus monkeys (Macaca mulatta) acquired from the same zoo and maintained on feed containing animal protein, developed a progressive neurological disorder with behavioural abnormalities and physical deterioration and died at the age of 10-year-old. Neuropathological examination of one of these animals revealed a spongiform encephalopathy similar to that observed in monkeys following experimental transmission of Creutzfeldt-Jakob disease (CJD). In particular, several brain regions exhibited vacuolation of nerve cell bodies and processes accompanied by astrogliosis. Immunohistochemical analysis showed prion protein (PrP) immunoreactivity at the periphery of vacuolated neurons. The spontaneous occurrence of a SE in these young monkeys might be related to consumption of protein of animal origin.