In the fat body of Aedes aegypti a very high unspecific esterase activity and a low lipolytic activity was found. The electrophoretic isozyme patterns of the unspecific esterases show only few changes in the different physiological stages. The activity of the unspecific esterases as well as of the lipase is especially high in young sugar fed and in blood fed mosquitoes which points to special energy requirements in these stages. The role of the unspecific esterases is discussed.