The subepidermal fat body of the Amphipod Crustacean Orchestia gammarellus shows ultrastructural modifications related to vitellogenin synthesis. In the adipocytes of vitellogenic females, the rough endoplasmic reticulum is well developed whereas in those of males and non-vitellogenic females it is almost entirely absent; lipids and glycogen are, on the contrary, less abundant. The unlabelled antibody enzyme method shows the presence of vitellogenin in the dense bodies of the adipocytes of vitellogenic females. Adipocytes of males and non-vitellogenic females are not immunoreactive.