Our previous study failed to show vitellin synthesis in the ovary of the prawnMacrobrachium rosenbergii(Sagi et al., 1995); thus the role of the hepatopancreas as a possible site of synthesis was evaluated. Extracts of hepatopancreas and hemolymph of a secondary-vitellogenic female exhibited higher levels of yolk protein than those from a primary-vitellogenic female. Clear vitellin immuno-cross-reactivity was observed in hepatopancreas sections from a secondary-vitellogenic female while no such reaction was found in a male hepatopancreas. Furthermore, vitellin-immuno-cross-reactive polypeptides released into the culture medium of the hepatopancreas of a secondary-vitellogenic female were similar to those found in the hemolymph and ovary (92 and 105kDa). The most prominent immuno-reactive polypeptide in the hepatopancreas extract was a relatively low-molecular-weight species (42kDa).De novosynthesis of cross-reactive-vitellin polypeptides (34, 38 and 42kDa) was detected in the hepatopancreas of a secondary-vitellogenic female. Synthesis of these polypeptides were not detected in the secondary-vitellogenic ovary or in the male hepatopancreas. The appearance of similar polypeptides following incubation of a secondary-vitellogenic ovarian extract with a glycosidase suggests that these polypeptides could be subunits of a core protein of vitellogenin, which was synthesized in the hepatopancreas and then released to the hemolymph following post-translational modifications. Our findings thus suggest the hepatopancreas to be a likely site of synthesis of a yolk protein precursor inM. rosenbergii.