Aedes triseriatus females collected as pupae from 2 tireyards and one woodlot were tested for susceptibility to La Crosse virus infection and ability to transmit the virus to suckling mice. Rates of disseminated infection and oral transmission were inversely correlated with mean body size. The smallest population developed disseminated infections and transmitted virus at significantly higher rates than the 2 larger populations. Adults derived from F1 eggs of the largest and smallest populations showed no significant differences in rates of disseminated infection and transmission, suggesting that the differences between the field populations were primarily caused by larval rearing conditions. When pupae of 2 strains of Ae. triseriatus were arranged into discrete size groups and tested for susceptibility to virus, an inverse relationship was observed between rate of disseminated infections and mean pupal wet weight. In contrast, Ae. hendersoni exhibited low transmission rates (7-10%) regardless of body size.