During the months of January to April, 1923, a study was undertaken of the hibernation of Anopheline mosquitos in Palestine. Previous experience indicated that the activity of these mosquitos was greatly redused during the winter months. It remained, however, to determine whether this inactivity was relative or absolute, that is, whether it was due to real hibernation or only to either retarded development of the immature stages or inhibited activity of the adults. Such information is, of course, of first importance in any attempt at intelligent control of malaria.