This paper investigates vitellogenin synthesis in the fat body of the female marine crustacean Isopoda, Idotea balthica basteri, during vitellogenesis. The fat bodies were incubated in a labelled medium; one of the samples was treated with an antiserum against vitellogenin and the antigen-antibody complex counted for radioactivity. We assumed this radioactivity to be due to vitellogenin synthesis. It is indicated that vitellogenin may account for most of the protein synthesized by the fat body during vitellogenesis. The first peak of vitellogenin synthesis was observed in the early stages of vitellogenesis (molting cycle period C), but the major increase in that level appeared during stage 2 of vitellogenesis (molting cycle period D, stage D1). Before laying (and molt), the fat body incorporated less.