A new polyhydroxylated lactone, discodermolide (l), was isolated from a Caribbean marine sponge, Discodermia dissoluta, and its structure was elucidated through a combination of spectroscopic techniques, in particular NMR spectroscopy, and verified by X-ray crystallographic analysis. The carbon skeleton displayed by discodermolide is new; discodermolide is immunosuppressive and cytotoxic.