Microginin, an angiotensin-converting enzyme inhibitory pentapeptide, was isolated from the freshwater blue-green alga Microcystis aeruginosa. Its structure was elucidated to be 1 on the basis of 2D NMR data and chemical degradation.Microalgae have recently received much attention as sources of novel bioactive compounds.*-5) Especially, blue-green algae have been shown to be excellent producers for cytotoxins and fungicides by Moore et al.6) In the course of our screening program of enzyme inhibitors from microalgae, we isolated an angiotensin-converting enzyme inhibitory peptide from the cultured freshwater bluegreen alga Microcystis aeruginosa (NIES-100). which is well known to produce hepatotoxic peptides microcystins.8) Angiotensin-converting enzyme inhibitors have been developed as antihypertensive agents.9) In this paper, we describe the isolation and structure elucidation of a linear pentapeptide microginin (1).