In this paper, we report on the structure and biological activity of a novel peptide, aeruginosamide, isolated from a bloom of the cyanobacterium Microcystis aeruginosa. The sample of M. aeruginosa was collected from a bloom that occurred on Rutland Water, U.K., in September 1989, and the crude methanolic extract was purified using C18 reversed-phase chromatography. Structure elucidation was performed using a combination of NMR techniques (1H, 13C, DEPT-135, selective one-dimensional TOCSY, gradient-filtered HSQC, gradient-filtered HMBC, T-ROESY) and mass spectrometry (low-resolution electrospray mass spectrometry, high-resolution electrospray mass spectrometry). The compound was found to be a modified peptide containing L-Pro, L-Ile, L-Val, a diisoprenylamine, and a unique proline-thiazole-methoxy ester moiety, representing a novel structural type. The absolute stereochemistry of the amino acid residues was determined by acid digestion followed by chiral TLC, confirming L-Pro, L-Ile, and L-Val. Aeruginosamide showed mild cytotoxicity to A2780 human ovarian tumor cells and K562 human leukaemia cells with ID50's of 2.9 and 5.2 µM, respectively.