Oscillatorin(1), 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. Oscillatorin included an abnormal amino acid unit, (3a-cis)-1,2,3,3a,8,8a-hexahydro-3a-(3-methyl-2-butenyl)-pyrrolo[2,3-b]indol-2-carboxylic acid.