Aeruginosins 98-A and B, trypsin inhibitors, were isolated from the cultured freshwater blue-green alga Microcystis aeruginosa. Their structures were elucidated to be 1 and 2 respectively on the basis of 2D NMR data and chemical degradation. These peptides inhibited trypsin potently with an IC50 of 0.6 μg/ml.