Maitotoxion, a putative Ca²⁺ channel agonist, was isolated from cultures of the marine dinoflagellate Gambierdiscus toxicus as a colorless amorphous solid. The toxin reacted positively to Dragendorff's reagent but not to ninhydrin reagent. The mouse lethality of maitotoxin determined by intraperitoneal injection was 0.13μ/kg. Chemical features of the toxin were elucidated mainly by various spectroscopic measurements. The molecular weight of maitotoxin as a disodium salt was estimated to be 3, 424.5±0.5 from the negative fast atom bombardment (FAB) mass spectrum. The presence of two sulfate ester groups in the molecule was apparent from the IR and mass spectra, and from analyses of solvolysis products. Despite of its large size, maitotoxin seems to have no known repeating units, such as amino acids and sugars, no carbonyl groups, no side chains other than methyls or an exomethylenes, and no carbocycles.