Evidence for the multiplication of insect-transmitted viruses in the insect host has now been obtained in three instances. Holt and Kintner (1931), using the feeding method of St. John, Simmons and Reynolds (1930), were able to pass dengue virus through three, but not through five, successive lots of mosquitoes. Using essentially the same method, Merrill and Ten Broeck (1934) passed western strain equine encephalomyelitis virus through more than ten successive lots of mosquitoes, and showed that the virus multiplied and retained all its characteristics. Merrill, Lacaillade and Ten Broeck (1934) found that Aedes sollicitans infected with the eastern strain of equine encephalomyelitis virus contained more virus five days after the infective feed than just after it. Whitman (1937), using mice for titration, has recently shown that yellow fever virus multiplies in Aedes aegypti. His work contradicts the earlier negative results of Davis, Frobisher and Lloyd (1933), obtained with titration in monkeys.