We report a mass-spectrometry-based metabolomics study of a laboratory-cultured strain of Microcystis aeruginosa (UTEX LB2385), which has led to the discovery of five peptides (1-5) belonging to the microginin class of linear cyanopeptides. The structures and configurations of these peptides were determined by spectroscopic analyses and chemical derivitization. The microginin peptides described herein are the first reported derivatives containing N-methyl methionine (1, 5) and N-methyl methionine sulfoxide (2-4). The two tripeptide microginin analogues (4, 5) identified represent the smallest members of this peptide family. Their angiotensin-converting enzyme (ACE) inhibitory activity was also investigated. Microginin 527 (4) was the most potent of the group, with an IC50 of 31 μM.