A recent publication (1) records the transmission of yellow fever to laboratory animals by the bite and by the injection of jungle mosquitoes which had acquired the infection in their native habitats. The present paper describes the methods used for capturing, transporting, classifying, and feeding the mosquitoes which were used in the experiments. The technique was developed and put into practice during an epidemic of jungle yellow fever in the vicinity of Maracajú, Mato Grosso, during the first six months of 1937. It was again used during the epidemic in the State of Rio de Janeiro, in the first half of 1938. The prime objective was to obtain yellow fever infection in laboratory animals by the bite of jungle mosquitoes within twenty-four hours after their capture. Captures were made only in jungle areas in which humans were known to have contracted yellow fever.