The object of this article is to describe techniques used for the study and maintenance of the virus of yellow fever in the laboratory at Villavicencio, Colombia. The characteristics of ye- Howfever virus are briefly described. The susceptibility of white mice of the Swiss strain is discussed, and the technique of their routine use for tests of the presence of virus described. The methods of virus titration, specificity tests and protection tests are explained in some detail. The susceptibility of Colombian monkeys to yellow fever virus and their use as laboratory animals in virus studies are described. The isolation of yellow fever virus from suspected cases of human infection is discussed and the technique of preserving virus by desiccation described. Methods of handling mosquitoes in the laboratory are described, with special reference to techniques adapted to transmission studies with Haemagogus mosquitoes.