Before the anti-stegomyia campaigns in Havana, Panama and Guayaquil, epidemics of yellow fever were not uncommon in the towns and cities of Colombia along the coast and in the Magdalena valley. Following the last known outbreak in a coastal town, which occurred in Buenaventura in 1920, the country appeared to be entirely free of the disease until the unexplained appearance of urban yellow fever in Bucaramanga in 1923. After six quiet years, the disease broke out unexpectedly again in Socorro in 1929. Immunity surveys carried out by the newly discovered mouse protection test in 1931 and 1932 gave evidence that yellow fever might be endemic in some rural areas of Boyacá and Santander. As a result of a systematic study which was begun in Colombia in 1934, cases of jungle yellow fever have been found each year since in zones comprised of the departments of Antioquia, Boyacá, Caldas, Cundinamarca, Santander and Tolima, and the Intendencia del Meta. The results of an extensive immunity survey during this period confirm this distribution of yellow fever and make it appear probable that the disease has existed in recent years in other widely separated parts of the country such as the Atrato valley and the Colombian Amazon drainage. With the exception of a very small epidemic in Buenavista, Caldas, in 1937, no incidence of yellow fever transmitted by Aedes aegypti has been observed in Colombia since 1929.