Six isolates of a new phlebotomus fever serogroup virus, designated Arboledas virus, were obtained from sand flies (Lutzomyia spp.) collected in northeastern Colombia. One of the isolates was made from a pool of male sand flies. By immunofluorescence, Arboledas virus is related to Caimito and Pacui viruses; by neutralization test, it is distinct. Arboledas virus neutralizing antibodies were found in the sera of opossums (Didelphis marsupialis) and humans living in the study area. D. marsupialis inoculated with the virus developed a viremia of four days' duration, and sand flies (Lutzomyia gomezi) feeding on a viremic opossum were readily infected. Transovarial transmission of Arboledas virus was also demonstrated in experimentally infected Lu. gomezi. Results of the above laboratory studies suggest that Arboledas virus is maintained in nature by two mechanisms: vertical (transovarial) transmission in the insect vector, and an alternating marsupial-sand fly cycle. The implications of this complex maintenance cycle for other phleboviruses are discussed.