Micropeptin 90, a plasmin and trypsin inhibitor, was isolated from the cultured freshwater blue-green alga Microcystis aerug/nosa (NIES-90). Its structure was elucidated to be I on the basis of 2D NMR data and chemical degradation. This cyclic depsipeplide inhibited plasmin and trypsin potently with ICs0 of 0.1 and 2.0 I.tg/ml, respectively.