Methyl farnesoate (MF) has been identified as a juvenile hormone-like compound in crustacea which has central roles in the regulation of development and reproduction. To study the regulation of MF synthesis, we isolated a neuropeptide which inhibits MF synthesis from the neurohemal organ-sinus gland X-organ complex of the spider crab Libinia emarginata. The primary structure of this neuropeptide has been determined. It has 72 amino acid residues (deduced molecular mass 8490.5 Da) with pyroglutamic acid at the N-terminus and NH3 at the C-terminus. It shares a high percentage of sequence identity with other sinus gland neuropeptids which form the unique family of CHH neuropeptides of crustacea. Activity studies showed that this neurohormone has dual effects: it inhibited MF synthesis in vitro and had hyperglycemic activity when injected into crabs.