Zooxanthellamide B, C(128)H(220)N(2)O(53)S(2), a polyhydroxy secondary metabolite, was isolated from a cultured marine dinoflagellate of the genus Symbiodinium. A detailed 2D NMR analysis revealed the chemical structure as a delta-lactone analogue of zooxanthellamide A, which had previously been isolated from the same dinoflagellate by us. The relative configuration of the delta-lactone moiety was determined by NOE experiments and a coupling constant analysis, and that of other ring systems was found to be the same as zooxanthellamide A by the chemical correlation between zooxanthellamides A and B.