Studies were conducted to determine the potential for transmission of a live, attenuated chikungunya (CHIK) virus vaccine by orally exposed or virus-inoculated mosquitoes. The vaccine (CHIK 181/clone 25) replicated in and was transmitted by female Aedes albopictus and Ae. aegypti after intrathoracic inoculation. Mosquitoes also became infected with the vaccine after ingesting virus from either a blood-soaked cotton pledget or a viremic monkey. However, because of the low viremias produced in inoculated humans, it is unlikely that mosquitoes would become infected by feeding on a person inoculated with the live, attenuated CHIK vaccine. Although the vaccine was transmitted by mosquitoes after intrathoracic inoculation, there was no evidence of reversion to a virulent phenotype.