Crustacean hyperglycemic hormone (CHH)-family peptides, Pej-SGP-I ~ -VII including molt-inhibiting hormone (Pej-SGP-IV), were purified from the sinus gland extract of the Kuruma prawn (Marsupenaeus japonicus) and their structures were elucidated. The cDNA’s encoding some of them were cloned, revealing that they were classified into two types, type I and II. Six peptides other than Pej-SGP-IV (molt-inhibiting hormone, MIH) showed both hyperglycemic and vitellogenesis-inhibiting activities. The neurosecretory cells producing these peptides were identified by mmunohistochemical methods. The three-dimensional structure of MIH was determined by NMR, clarifying the spherical structure stabilized by five α-helices, three pairs of disulfide bonds, and a hydrophobic core. The receptor-binding site of MIH could also be estimated. cGMP was identified as a second messenger in the MIH signal transduction. We also tried to identify an MIH receptor in the Y organs, molting hormone producing organs, but could not yet. It was found that MIH inhibited the expression of at least one enzyme involved in the ecdysteroid biosynthesis. CHH family peptides were also identified from other crustacean species.