As observed by us the duration of the intrinsic incubation period of naturally inoculated vivax malaria does not vary within wide limits. In a series of 182 successful inoculations the number of days elapsing from inoculation to the first microscopical detection of parasites has ranged from 8 to 23 in 180 instances. There are two white cases in our series for which incubation periods of 85 days (case 108) and 97 days (case 106) have been provisionally recorded. These patients were inoculated on December 15 by three mosquitoes each. The same mosquitoes were applied to two other white patients on December 20, and on subsequent dissection were found positive. The two latter had incubation periods of 10 and 15 days respectively. Since the infection was not detected in the first two after a month's time, other mosquitoes were applied to one (no. 108) on February 24, and the same mosquitoes to the other on March 4, when they were not observed to feed.