The most important characteristics and the distribution of the viruses of African swine fever and hog cholera are reviewed. Both viruses were probably present simultaneously in South Africa in the first two decades of the century. While hog cholera was eradicated by 1918, African swine fever persists to the present day because it has a different epizootiology. The role played by wild pigs and the argasid tick (Ornithodoros moubata porcinus) in the epizootiology of African swine fever is discussed and an account of the outbreaks of the disease in South Africa from 1926 to 1974 is given. It appears that the disease in the Transvaal has had a cyclic occurrence.