Non-toxic mussels Mytilus galloprovincialis were fed for 5 days with a natural population of the toxic dinoflagellate Dinophysis fortii. The total number of D. fortii cells ingested by five mussels during the feeding experiment was approximately 234 x 10(3). Okadaic acid (OA), dinophysistoxin-1 (DTX1) and esterified toxins of OA and DTX1 in D. fortii and mussel extracts were determined by liquid chromatography followed by on-line atmospheric pressure electrospray ionization-mass spectrometric (ESI-MS) detection. DTX1 was the only derivative detected in extracts of the D. fortii cells used for the feeding experiment whereas mussels fed on D. fortii contained both DTX1 and esterified DTX1. DTX1 was present in significantly higher amounts in the mussels than esterified DTX1. The absorption efficiency of DTX1 in the midgut glands of mussels was approximately 9 % of the total amount of DTX1 in D. fortii cells filtered by the mussels. The absorption efficiency is higher than that reported previously in scallops (less than 3%).