Dinoflagellates of the genus Dinophysis produce okadaic acid (OA) and analogues. Cultures of a strain of Dinophysis acuta isolated from the Galician Rías contained high levels of pectenotoxins but little OA. Isomeric forms of the latter were suspected as the concentration of OA increased after alkaline hydrolysis. High resolution mass spectra of candidate compounds suggest the presence of an OA-C9-diol ester, reported for the first time in Dinophysis acuta.