Abstract A 62-membered novel polyol macrolide with a molecular weight of 2859 mu, symbiodinolide, was isolated from the symbiotic dinoflagellate Symbiodinium sp. Symbiodinolide exhibited a potent voltage-dependent N-type Ca 2+ channel-opening activity at 7 nM and immediately ruptured the tissue surface of the acoel flatworm Amphiscolops sp. at 2.5 μM. The planar structure of symbiodinolide was elucidated by spectroscopic analysis and chemical degradations including hydrolysis and ethenolysis using the second-generation Grubbs' catalyst. Symbiodinolide was found to be a structural congener of zooxanthellatoxins. The relative stereochemistries of C26–C32, C44–C51, and C64–C66 parts, and the absolute stereochemistries of C69–73, C83–C103, and C3′–C18′ parts in symbiodinolide were established.