Summary and Conclusions 1. Two strains of jungle yellow fever virus have been transmitted by the bites of Aëdes aegypti under laboratory conditions. The M. A. J. strain has been carried through 17 mosquito-monkey-mosquito passages and the Suarez strain through four. No difficulty has been encountered in maintaining the passages. 2. The minimal incubation period between the infectious meal and the earliest capacity to transmit by bite in Aëdes aegypti has been shown to be about fourteen to fifteen days, when the M. A. J. strain is used. This is several days longer than has been found necessary for the Asibi strain, a virus of urban origin. 3. It is suggested that the prolonged incubation period may play a part in limiting the importation of jungle virus into towns, since, with a lengthened period, a greater percentage of infected insects would die before becoming infectious. 4. Of 27 monkeys infected with the M. A. J. strain, 13 of 17 survivors either had no fever, or first showed fever after the virus had practically disappeared from the blood stream. In the four remaining survivors and in the 10 animals which succumbed, the onset of fever corresponded with the period of maximum circulating virus. 5. It is suggested that the delayed febrile response in non-fatal cases may not be a characteristic of jungle virus, as such, but may be related to the pathogenicity for rhesus monkeys of the particular strain of virus used, whether of urban or jungle origin. It is further suggested that, if the human response is comparable to that of rhesus monkeys, many cases brought to town for treatment after the onset of fever would no longer be infectious for the local Aëdes aegypti. This would reduce the frequency with which the jungle virus might invade urban centers.