In this study, variations in rate of protein, glycogen and water of cotton leafworm Spodoptera littoralis (Boisduval) (Lepidoptera: Noctuidae) larvae which spread widely in warm regions and Mediterranean countries causing economic losses of some agricultural products were investigated during consecutive development stages. It was determined that statistically significant variations at rate of protein, glycogen and water of the larvae in different levels during the larval stages. Especially, it was found that a remarkable reduction in the rate of larval glycogen from fifth larval stage. Similar decrease was observed for the rate of water from fourth instar. Higher values were noteworthy in the rate of larval protein about first and last instar than the other stages. Larvae may have stored a number of protein in the final larval stage to be used in the pupal stage. It was not determined a statistically significant relation between the water and protein content (r(2) = 0.049). Similarly, it was observed that a remarkable decrease for glycogen content of the larvae in the last stages but it could not be identified compatible changes with this reduction in protein content of the larvae (r(2) = 0.008). Unused protein of larvae has likely been excreted with feces without causing any chemical transformation. On the other hand, the reduction of larval glycogen approaching to the pupal stage supported that the idea of glycogen is not only used as an energy source but also used for the formation of imaginal tissues and pupal cuticle in the process of transformation from larvae to adult.