Vertebrate viruses isolated from phlebotomine sandflies belong to three distinct arbovirus serogroups: vesicular stomatitis, Phlebotomus fever, and Changuinola. In addition to their antigenic dissimilarities, the physicochemical properties and morphologic characteristics of these sandfly-associated viruses are also quite different. However, one feature that they share in common, in contrast to most other vertebrate arboviruses, is their apparent inability to produce a significant viremia in infected animals or man. This characteristic suggests that vertebrates may be dead-end hosts for most sandfly viruses and that maintenance mechanism other than the classic vertebrate-insect cycle must be postulated to explain their biologic survival. Accumulating evidence indicates that insect-to-insect (transovarial) transmission is the maintenance mechanism. In the present paper, we shall examine the field and laboratory evidence in support of this hypothesis and will present new data on the transovarial transmission of vesicular stomatitis virus, Indiana serotype (VSV-Indiana), by sandflies.