Canine distemper virus (CDV) has been implicated in some recent deaths of lions, which showed clinical signs of distemper, in the the Serengeti plain. Similar clinical findings have since been reported in lions of the Masai Mara. Fifty-five per cent of serum samples obtained from wild lions of the Masai Mara have been found to contain neutralising antibody to CDV, indicating that they had been exposed to the virus. Adult orphan lions kept in captivity, were vaccinated with the live attenuated Onderstepoort strain of CDV. The results indicated that the vaccine is both safe and immunogenic, and may be potentially useful for the prophylactic vaccination of lions at high risk.