Contamination of bivalves with yessotoxins (YTXs) has been reported since 1987 in several coastal environments. In our study we investigated the accumulation and the metabolism of YTXs in 2 bivalve species important to German fisheries and aquaculture. Mussels and oysters, Mytilus edulis and Crassostrea gigas, were exposed to the YTX-producing dinoflagellate Protoceratium reticulatum isolated from the North Sea. Toxin profiles from algae and bivalves were analyzed by liquid chromatography with tandem mass spectrometry (LC-MS/MS). It was evident that 45-hydroxy YTX (45-OH YTX), carboxy YTX (COOH YTX), and the putative 45-hydroxy-carboxy YTX (45-OH-COOH YTX) were the dominant YTX analogues in M. edulis. The oyster C. gigas displayed a different toxin profile. YTX, 45-OH YTX, and 45-OH-COOH YTX were also present, but COOH YTX was not detectable. Furthermore, 2 unidentified analogues with the same MS/MS transition ([M-H] > [M-H-SO₃]) occurred in both bivalve species. Homo YTX and 45-hydroxy-homo YTX, which are regulated together with YTX and 45-OH YTX in 2002/225 EC were not detected. In general, we confirmed former field investigations on accumulation and metabolism of YTXs in M. edulis; however, this is the first study on accumulation and metabolism of YTXs in the oyster C. gigas.