Summary and Conclusions 1. The quantity of virus in the circulation of a vaccinated human being is too small under most circumstances to permit the infection of Aëdes aegypti. 2. Of eight human volunteers fed on by Aëdes aegypti on the 6th and 7th days after vaccination, two had minimal amounts of virus on the 7th day. The mosquitoes feeding at that time were not infected. 3. Of twelve lots of mosquitoes fed on rhesus monkeys after vaccination, five lots engorged at a time when the concentration of virus in the blood stream was as great as or greater than that observed in a carefully studied group of twenty-nine human beings after vaccination. None of the five lots was infected. 4. Four attempts were made to infect Aëdes aegypti in the larval stage by immersing them in large quantities of vaccine virus. The adults emerging from larvae infected in the two more concentrated virus preparations were shown, by injection into mice and monkeys, to be infected. However, neither lot was capable of transmitting the virus by bite. In one case thirty-five insects failed to transmit by bite, though the identical mosquitoes were shown to contain the virus in their bodies. In the other case twenty-nine insects gave a similar result. None of the three monkeys infected by the injection of the mosquitoes had fever, nor was there any increase in circulating virus as a result of the single passage through the bodies of the mosquitoes. 5. It is concluded that the danger of Aëdes aegypti transmitting the 17D vaccine virus is at best slight, and, in all probability, nonexistent.