The toxic profile of dinoflagellates varies even among identical species, raising an interesting question about the mechanism of toxin regulation and production. In consequence, it also poses a considerable problem in their control methods. In this paper, we report on the isolation and structural elucidation of several new ester derivatives of okadaic acid (OA) from artificial cultures of the genus Prorocentrum. These new compounds enlarge the range of target molecules that must be considered in the monitoring programs.