Abstract The protein contents in the larval haemolymph of Culex pipiens (autogenous form) and Culex fatigans (anautogenous form) have been determined by paper electrophoresis. In both mosquito strains the electropherograms showed only one protein fraction which increased rapidly in its concentration during the later period of larval development. The fully grown larva of the anautogenous form was found to contain more proteins per unit volume blood than that of the autogenous form. On the other hand, the total blood concentration of the free ninhydrin-reacting components, as estimated by the method of paper chromatography, is higher in the autogenous form than in the anautogenous form. On the basis of data presented in this study, the relation of nutritional reserves to the phenomenon of autogenous egg production in mosquitoes is discussed.