Okadaic acid (OA) is the main toxin produced by dinoflagellates, which can accumulate in the hepatopancreas of mussels and cause diarrhoetic shellfish poisoning in consumers. This toxin is also a tumour promoter and a specific potent inhibitor of protein phosphatases 1 and 2A. The results in this study show for the first time that this marine toxin is able to cross the transplacental barrier. Foetal tissue contains more okadaic acid than the liver or kidney: 5.60% compared to 1.90 and 2.55% respectively as measured by HPLC and fluorescent detection after derivatization with 9-Anthryldiazomethane (ADAM). In view of its adverse effects, okadaic acid might impair foetal development and promote tumours in neonates.