Summary 1. The reported incidence of yellow fever is no safe index of its occurrence in endemic zones. 2. Although visible urban and maritime outbreaks may decline and even cease entirely for a time, there is a vast, previously silent reservoir of infection in the interior of South America. 3. Yellow fever infection due to Aëdes aegypti has been much more widespread in the interior of Northeast Brazil than was believed, even though this area had long been under special observation. Aegypti-transmitted fever in this area did not spontaneously disappear following the organization of antiaegypti campaigns in the principal centers of population. 4. Yellow fever endemicity, instead of being limited to the coast of Northeast Brazil as was believed, extends to all of Brazil except a few of the southern states, to Bolivia, Paraguay, Peru, Ecuador, Colombia, and Venezuela, involving many districts in which Aëdes aegypti does not exist. 5. Widely varying percentages of immunes have been found in proved endemic regions, depending upon whether transmission is due to Aëdes aegypti or occurs in the absence of this mosquito. 6. There is no evidence of recent yellow fever outbreaks in any of the important Pacific or Caribbean ports of South America.