Oscillamide Y, a chymotrypsin inhibitor, was isolated from freshwater toxic cyanobacterium Oscillatoria agardhii. The structure was elucidated to be 1 by chemical degradation and 2D NMR analyses.The toxic strain of freshwater cyanobacterium(blue-green algae) Oscillatoria agardhii forms waterblooms in freshwater lakes and drinking water reservoirs, and produces heptacyclic peptide hepatotoxins named microcystins 1.During investigations of cyclic peptide toxins of the toxic strain of O. agardhii, we found a novel chymotrypsin-inhibiting cyclic hexapeptide. We now describe the isolation and structure elucidation of 1.