With the crossing of the yellow fever epizootic wave from Honduras to Guatemala at the end of 1955 a new ecological area with unknown vectors became involved. Starting in March and continuing until three negative months had been recorded, mosquitoes were captured in the lower Motagua Valley and sent to the Gorgas Memorial Laboratory in Panama where attempts at virus isolation were made by inoculation of species lots into various mammals. During the course of the studies 25 viruses were isolated—22 of yellow fever and 3 of other viruses. As a result Haemagogus mesodentatus was shown to be the prime vector with H. equinus and possibly Sabethes chloropterus as secondary transmitters. The epizootic wave after decimating the primates of the lower Motagua Valley has continued across the Lake Izabal area and is now headed for Petén and Alta Verapas, areas suitable for the continuation of the disease into southern Mexico, where favorable conditions for sylvan yellow fever exist.