Lipid is the predominant organic reserve of many crustaceans and is important in the metabolism of many of these animals. Ingested lipid is digested by gastric lipase and apparently absorbed into depot-lipid as rβ-monoglycerides. The variation in the content and composition of the depot-lipid is a function of both the external environment and internal control systems. Evidence suggests that lipids from marine organisms contain more long-chain polyunsaturated fatty acids than does the lipid of fresh water organisms which in turn have a high proportion of C16 and C18 fatty acids. The fatty-acid composition of the sub-tropical land crab,Gecarcinns lateralis, resembles that of the fresh-water crustaceans. In addition, our studies indicate that aspects of lipid metabolism may be under endocrine control. The induction of premolt by destalking markedly increases the synthesis of lipid from metabolic precursors and its subsequent incorporation into the depot-lipid of the hepatopancreas. In the late premolt stages there is a decrease in the lipid content of the hepatopancreas. This occurs as the lipid is mobilized from the hepatopancreas to meet the energy demands of all those processes resulting in ecdysis. This sinusoidal variation in the lipid metabolism of the hepatopancreas is influenced by an eyestalk factor (s).