Vitellogenin synthesis was investigated in the ixodid tick,Ixodes scapularis.The fat body was the most active tissue in vitellogenin production. Midgut and ovarian tissues showed slight activity (~2000 cpm) which may have been due to a small amount of contamination by fat body cells. Proteins synthesized by the fat body were examined and confirmed as vitellogenin by electrophoresis and audioradiography. There were seven radioactive polypeptides corresponding to the subunits of vitellogenin. Immunochemical analysis showed a strong reaction with anti-vitellin antibody in the fat body and developing ovaries; however, no specific fluorescence was seen in the midgut. These results indicate that the fat body is the main and probably the only site of vitellogenin synthesis in this prostriate tick.