The structures of hepatotoxins cyanoginosins-XY from Microcystis aeruginosa are proposed as cyclic heptapeptides: Cyclo-D-Ala-L-X-erythro-β-methyl-D-isoAsp-L-Y-Adda-D-isoGlu-N-methyldehydroAla, where X and Y represent variable amino acids and Adda is 3-amino-9-methoxy-2,6,8-trimethyl-10-phenyldeca-4,6-dienoic acid. Using NMR and mass spectral methods (including fast-atom bombardment mass spectrometry, gas chromatography-mass spectrometry), we report full or partial structures of four analogues (cyanoginosin-LR, -YR, -YA, and -YM) — three from a South African source and one from Malpas Dam, Australia. Amino acid analyses and stereospecific studies showed these toxins contain D-amino acid residues (erythro-β-methyl-D-isoAsp, D-isoGlu, D-Ala), N-methyldehydroalanine (N-Medha), and variable L-amino acids (Leu/Arg for -LR, Tyr/Arg for -YR, Tyr/Ala for -YA, Tyr/Met for -YM). Molecular weights (e.g., -YR: 1044 Da, -LR: 944 Da, -YM: 1035 Da) and cyclic structure (absence of sequence ions in mass spectra) were determined. For -YM, the D-configuration of β-MeAsp was confirmed by GC-MS, N-Medha presence by reduction/hydrolysis, and oxidized Met (sulphoxide) by 2-mercaptoethanol treatment. The presence of Adda in all four toxins was strikingly demonstrated by NMR (matching chemical shifts/coupling constants of cyanoginosin-LA) and mass spectral fragments. This study clarifies the structural features of these cyanobacterial cyclic peptide toxins, including key unusual residues and variant amino acids.