A marine sponge Halichondria sp. contained halichondramide (4) as the major constituent and dihydrohalichondramide (6), isohalichondramide (10), acid 11, imide 12, and ester 13 as minor metabolites. The compounds isolated from the nudibranch Hexabranchus sanguineus varied depending on collecting location but usually included dihydrohalichondramide (6) and tetrahydrohalichondramide (8). The macrolides 4,6,8, and 10 have antifungal activity and inhibit cell division in the fertilized sea urchin egg assay. We now report the structural elucidation of halichondramide (4) and six related metabolites from a sponge of the genus Halichondria and specimens of the nudibranch H. sanguineus both collected at Kwajalein Atoll.