Normal Aedes aegypti males were exposed to infected A. aegypti females immediately, three weeks, and nine weeks after the females had taken infecting blood meals. In a fourth experiment, infected females were allowed to engorge upon a normal monkey just before the males were liberated with them. In 2 experiments the females could be seen expelling droplets of clear fluid at the time the males began to copulate with them. The males were exposed to the females for from one to nine days. The infectivity tests performed from one to thirty-two days later with 152 of these males which had been exposed under a wide variety of conditions, gave no evidence to indicate that any of the males contained yellow fever virus. The results of the immunity tests confirmed the results of the infectivity tests. Of the 18 monkeys so tested, 14 died of yellow fever, 3 more survived after febrile reactions which were interpreted as manifestations of attacks of yellow fever, and 1 monkey gave an inconclusive result.