After intrathoracic injection of 3.2 dex Bunyamwera (BUN) virus into groups of wild caught Aedes vexans and A. canadensis, increments of infectivity were first detected in salivary glands and gut at 3 days and maximum titers of 5.2 dex per organ were attained in the salivary glands at 10 days. Virus transmission by mosquito bite was demonstrated in suckling mice 10 days after the mosquitoes were injected, but not after shorter intervals. Virus replication was not demonstrable after the ingestion of 4.0 dex in a blood meal.Aedes aegypti mosquitoes readily supported the replication of BUN virus following injection with 3.3 dex or imbibing of 4.6 dex. After injection virus titers of whole mosquitoes declined to 1.7 dex at 12 h, followed by a peak of 5.2 dex at 4 days. After feeding, virus was first detected in the hemolymph and salivary glands at 4 days and attained maximum titers of 5.0 dex in salivary glands at 10 days. Transmission of virus to mice was accomplished 10 days after feeding, but not earlier.Enveloped viruses with 45 nm cores and 80 to 120 nm external diameter were observed within vacuoles and lining vacuolar membranes of salivary glands and gut cells of A. aegypti mosquitoes 10 days or more after infection with BUN virus. The dimensions and appearance of these particles correlated well with those of virions seen in chick embryo fibroblasts during the peak of the BUN virus infection cycle.