Liquid chromatography (HPLC) was used to search for esters of DSP toxins in Portuguese bivalves. Hexane-soluble derivatives of okadaic acid (OA) and dinophysistoxin-2 (DTX-2) were found. Presumably they are acyl derivatives, globally known as 'dinophysistoxin-3' (DTX-3). In certain instances DTX-3 content surpassed 50% of the total amount of DSP toxins. A human diarrhetic poisoning (DSP) incident with Donax clams (Donax trunculus) harvested at Fuzeta (Algarve coast) was confirmed where the apolar (DTX-3 type) and other esters remaining in the polar aqueous methanol layer were implicated. The percentage of acyl esters of OA was always higher than those of DTX-2. An enzymic mechanism for the conversion of OA and DTX-2 seems to be implicated in some kind of detoxification process because the percentage of esters increases with the toxin amount ingested by the bivalve and there is some degree of selectivity as DTX-2 seems more difficult to acylate. These findings pose a problem for the current assay methods used to detect DSP because mainly they are able to detect the parent toxins but not their esters. The current bioassay method [Le Baut, C., Bardin, B., Bardouil, M., Bohec, M., Masselin, P., Truquet, P., 1990. Etude de la decontamination de moules toxiques. Rapport IFREMER DERO-90-02 MR. 21 pp.] used in Portugal includes a hexane washing step that prevents interference from free fatty acids. However, it cannot detect the presence of acyl derivatives because they are highly soluble in hexane.