Two cyclic peptides, anabaenopeptins A (1) and B (2), were isolated as a third group of bioactive compounds from Anabaenaflos-aquae NRC 525-17. Their structures were determined by 2D-NMR techniques and Marfey's method combined with LC/MS.Cyanobacteria produce numerous secondary metabolites that are not used for primary, metabolism, l Since the 1970's many types of compounds have been isolated and they show characteristic biological activities including cytotoxic, imuunosuppressive, antifungal, cardioactive and enzyme inhibitory. In addition, several compounds of these metabolites produced by cyanobacteria show characteristic acute toxicity, which are classified into two groups according to their site of action. Microcystins, nodularin and cylindrosperrnopsin are potent hepatotoxins, while anatoxin-a, anatoxin-a(s) and aphantoxins (saxitoxins) are neurotoxins. 2 One strain of Anabaenaflos-aquae called NRC 525-17 is a unique cyanobacterium that can simultaneously produce anatoxin-a(s) and microcystins.3, 4 In this communication, we report the isolation and structural determination of a third group of compounds, anabaenopeptins from this cyanobacterial cells.