Our surveys indicate that nearly all batches of vervet monkeys arriving at the National Institute for Virology from various areas of South Africa, are infected with foamy viruses and there is evidence that intra-urterine infection also occurs. Monkeys from certain areas of South Africa are apparently infected with the protozoal parasite Entopolypoides macaci in a sub-clinical state which becomes active when the monkeys are splenectomised. Serological studies indicate that infections with schistosomes, tick-bite fever (Rickettsia conori), chlamydsiae and occasionally by leptospirae, occur in monkeys in the Kruger National Park. Complement fixation tests for antibodies to R. mooseri and R. prowazeki were also sometimes positive; a finding of doubtful importance. There was a high percentage with positive antibody tests to chikungunya virus in a certain age group suggesting that there had been an outbreak of this disease in the Kruger Park in the late summer and autumn of 1976. This coincided with a human outbreak. Large numbers of vervet and baboon sera were tested for antibodies against SA 12 virus, a polyoma virus. A high percentage of baboons but only a small number of monkeys was found to be positive and it is concluded that SA 12 is probably a baboon virus which is occasionally transmitted to monkeys by contact. It is pointed out that these studies only indicate diseases from which monkeys recover and they do not indicate prevalence of severe disease causing grave illness or death.