Oscillapeptin, a new cyclic depsipeptide that inhibits elastase and chymotrypsin, was isolated from the cyanobacterium Oscillatoria agardhii (NIES-204) and its structure was elucidated by extensive 2D NMR and chemical degradation. Oscillapeptin inhibited elastase and chymotrypsin with an IC50 of 0.3 and 2.2 μg/ml, respectively.