Okadaic acid (OA) homologues in toxic dinoflagellate Dinophysis fortii, scallops Patinopecten yessoensis and mussels Mytilus galloprovincialis collected at the same site in Mutsu Bay, Japan were determined by high-performance liquid chromatography fluorescence detection (HPLC-FLD) as their 9-anthryldiazomethane (ADAM) derivatives. Prominent toxin in the scallops and the mussels was esterified dinophysistoxin-1 (DTX3) and dinophysistoxin-1 (DTX1), respectively, although only DTX1 was detected in D. fortii. Toxin contents in the mussels were significantly higher than those in the scallops, indicating that mussels have higher potential to accumulate OA homologues than scallops.