Micropeptin 103, a chymotrypsin inhibitor, was isolated from the cultured freshwater cyanobacterium Microcystis viridis (NIES-103). So far, this type of peptide has not been reported from Microcystis viridis. Its structure was elucidated to be 1 on the basis of 2D NMRdata and chemicaldegradation. Micropeptin103inhibitedchymotrypsinwith IC50of 1.0pg/mL.