Spiroidesin (1), a novel D-amino acid-containing linear lipopeptide, was isolated from waterblooms of the cyanobacterium Anabaena spiroides. The structure was identified by 2D NMR and chemical degradation analyses. Spiroidesin inhibited cell growth of the toxic cyanobacterium Microcystis aeruginosa (IC(50), 1.6 x 10(-6) M).