Russian spring-summer virus encephalitis has been recognized in man in parts of the European and Far Eastern U. S. S. R. and in a few countries of Eastern Europe. Its viral agent is very closely related to, and possibly identical with, that of louping ill of sheep, long recognized in Scotland. Nonfatal infections in laboratory workers with the virus of louping ill have occasionally occurred.* Both viruses are known to be transmitted in nature only by ticks. Naturally acquired Russian spring-summer virus infections usually occur in forest workers bitten by ticks that have acquired their infection from small rodents. Although small rodents are the mammalian hosts, the tick is considered the true reservoir, inasmuch as the virus can be passed congenitally through the egg. Only three cases of the human disease due to the Russian virus have been recorded outside of Russian and Asian areas. Two were nonfatal laboratory infections