A new hepatotoxic microcystin was isolated from the cyanobacterium Microcystis aeruginosa strain 205. Its structure was found to be [Dha7]microcystin-RR as determined by amino acid analysis, mass spectrometry and 1H NMR spectroscopy. LD50 value (i.p. mouse) of this toxin was 180 micrograms/kg. The 48 hr lethal concentration (48-hr-LC50) of the toxin for larvae of the yellow fever mosquito, Aedes aegypti, was 14.9 micrograms/ml.