The growth-controlling neuroendocrine light green cells of the freshwater snail, Lymnaea stagnalis, express a family of genes encoding structurally related, yet distinct, molluscan insulin-related peptides (MIPs). In the present study one of these peptides, MIP II, has been isolated and structurally identified. MIP II is a heterodimer of A and B chains connected by disulfide bonds. Both chains are N-terminally blocked with pyroglutamate. After cleaving of the A and B chains and deblocking with pyroglutamate amino-peptidase their sequences have been determined as: A chain: pQRTTNLVCECCFNYCTPDVVRKYCY and B chain: pQSSCSLSSRPHPRGICGSNLAGFRAFICSNQNSPS. In comparison with the MIP II sequence based on complementary DNA studies, it is clear that the two C-terminal amino acid residues of the B chain are posttranslationally removed. In addition, the glutamic acid residue in A chain was recovered in very low yields during Edman degradation, suggesting that the residue may be posttranslationally modified.