Aeruginoguanidines 98-A–98-C (1–3) were isolated from the cyanobacterium Microcystis aeruginosa (NIES-98). The structures of aeruginoguanidines 98-A–98-C (1–3) were determined by two-dimensional 1 H–1 H and 1 H–13C NMR correlation experiments and confirmed by mass spectral analysis. The absolute stereochemistry of 1, consisting of Hphpa trisulfate (1-(4-hydroxy-3 hydroxymethyl)phenyl-1-hydroxy-2-propylamine 40 ,30 ,1-tri-O-sulfate), MpArg (N a-methyl-N v-prenylarginine) and MgArg ((Z )-N amethyl-N v-geranylarginine), was determined by the NMR analyses of phenylglycine methyl ester (PGME) or Boc phenylglycine (BPG) derivatives of acid hydrolysates. These compounds showed moderate cytotoxicity against the P388 murine leukemia cells.