Insects growth and development requires that their hard exoskeleton is shed periodically, and a new cuticle is synthesized. The soft new cuticle must become sclerotized, to protect internal tissues and to provide support for muscle attachment. The neurohormone bursicon initiates this sclerotization. Despite its importance, bursicon had remained poorly understood since its discovery in 1935. We showed that bursicon is a 30 kDa protein in different insects and is bioactive as a dimer. We retrieved partial amino acid sequences of highly purified cockroach bursicon. Antisera against one partial sequence identified the cellular location of bursicon in different insects. It is co-produced with crustacean cardioactive peptide (CCAP) in the same neurons in all insects we have tested so far. The complete sequence of bursicon was identified from Drosophila based on homology to the partial peptides. It is a monomer of about 15 kDa and a member of the cystein knot family that includes NGF TGFß and mucin. Using Drosophila as a tool, we have verified the function of the sequence in transgenic and mutant flies. Sequence comparisons also alloweed us to verify that bursicon is conserved, corroborating older data from bioassays and immunohistochemical results.