The biosynthetic origin of amphidinolide J 1 is investigated by means of feeding experiments of labeled [1-C-13], [2-C-13] and [1,2-C-13(2)] sodium acetate with cultures of the dinoflagellate Amphidinium so. which reveal that amphidinolide J 1 is generated through non-successive mixed polyketides, this may explain the generation of the unusual odd-numbered macrocyclic lactone ring.