Egg vitellin of the terrestrial isopodOniscus asselusis a 290 kD, female-specific lipoglyco-protein (V1) comprising two subunits, A (98 kD) and B (82 kD). An additional related protein (V2, 180 kD) was found in developing oocytes but not in mature eggs. Vitellogenin (VG) is electrophoretically and immunologically indistinguishable to V1and was detected in oocytes, fat body and haemolymph of vitellogenic females.In vitroincorporation studies revealed that VG is synthesized by both fat body and ovaries, but by no other tissues of either females or males.OniscusVG, therefore, has both intra- and extra-ovarian origins. Following long-term incorporation, the VG synthesized by either fat body or ovaries comprises only subunits A and B of V1. However, following short-term incorporation, larger polypeptides are present in these tissues, which are also recognized by an antiserum raised against V1. It is speculated that VG inOniscusis synthesized as a large polypeptide which is subsequently processed to smaller fragments. Synthesis of VG by explants of fat body was studied throughout the intermoult cycle. A single large peak of VG synthesis was seen in the second half of premoult; during this time, VG comprises 40% of the total protein synthesized by fat body and oocytes increase in volume 100-fold. The chronology of VG synthesis and oocyte growth are related to known times of release of hormones inOniscus.It is suggested that vitellogenesis is regulated by both ecdysteroids and neuropeptide(s) from the sinus gland.