There are conspicuous differences in respect of hibernation among the mosquitoes; for certain species, such as Culex pipiens and Anopheles maculipennis, hibernate as adults, but others, for instance A. claviger (bifurcates), pass the winter as larvae. These differences have been the subject of many investigations and of much theoretical discussion; perusal of the literature suggests that we still lack a sufficient knowledge of the facts. It is known that an accumulation of fat in autumn is characteristic of those mosquitoes which hibernate as adults, as it is of several insects of other orders. But it seems that little is known of the quantity of fat stored, or of the rate at which it disappears as winter passes. It is my present purpose to set out certain observations on the composition of adult female Culex pipiens at different times during the period of hibernation. Later it may be possible to make an experimental study of the causes of hibernation and to discuss the relevant literature. But at the moment let it suffice to refer to the following papers, from which the reader may inform himself of the present state of knowledge and theory: de Boissezon (1930), Roubaud (1932), and Shlenova (1933). The paper by Hecht (1933) contains a valuable summary of the subject.