During recent years much information has been gathered regarding the importance of wild animals in the epidemiology of sylvan yellow fever in Africa. That relating to the area in which the present work has been done is reported by Haddow, Smithburn, Mahaffy & Bugher (I 947) and their main findings may be summarized briefly here. Immunity to yellow fever was found to be widespread among the monkeys of the lowland Semliki forest but non-existent in any of the many other animal groups investigated. Although the several species of monkey studied had various day-time habits, some keeping purely to the trees and others spending more or less time on the ground, no significant differences in the immunity rates correlated with these particular differences were detected. It was deduced, therefore, that, if a single main vector insect were concemed, all species must be infected at a time and in a place in which they all adopt a like habit. Such a condition is fulfilled at night, as most species sleep in trees during this period. Entomological investigations in the same locality, devoting particular attention to the vertical distribution and daily rhythm of biting by forest insects