The structure and formation of the cuticle of Gammarus pulex L. is described and is found to be basically similar to that of decapod Crustacea. A scheme for designating stages in the moulting cycle is given. The principal food reserve is lipoid which is stored in the digestive gland and extensive adipose connective tissue. Glycogen is stored only in slight quantities, if at all, in intermoult animals, but appears in connective tissues, muscle sarcoplasm and epidermis during pre-moult. During pre-moult the muscle sarcoplasm forms an additional site for storing lipoid, which also appears in the epidermis. Following a moult the quantities of lipoid in muscle sarcoplasm and epidermis, and of glycogen in all sites, decline as the new cuticle approaches completion. Calcium is not stored. Animals fed on an enriched diet will store glycogen in all storage sites during intermoult. It is concluded that the lipoid and glycogen appearing in the epidermis are utilized in the formation of the new integument. The possible function of the tegumental glands in the Peracarida is discussed.