LaCrosse (LAC) virus, an arbovirus of the California group, will be further studies in its natural hosts the mosquito Aedes triseriatus and the chipmunk Tamias striatus to determine if either host selects for plaque variants. Viremic blood from experimentally infected chipmunks will be fed to mosquitoes. Total virus populations and selected plaque size variants of virus from both hosts will be compared for differences in mouse neuroviralence, mosquito infectivity, temperature stability and antigenic character. Plaque size, neurovirulence and antigenic character of LAC virus isolates from different areas will be compared to determine if this virus is geographically variable. Studies on medium-sized mammals as potential maintenance or disseminating hosts of LAC virus will include field surveys to detect naturally acquired antibody. Red foxes (Vulpes fulva) will be placed in enzootic foci as sentinels to determine frequency of natural infection. Foxes will also be tested for susceptibility to oral infection by LAC virus. Chipmunks and snowshoe hares (Lepus americanus) will be inoculated with 1) LAC virus, 2) the closely-related snowshoe hare virus, & 3) reassortants (derived from co-infection of cells with both agents). This experiment will determine if the reassortants can infect the natural hosts of the parent viruses.