An amperometric sensor system for measuring okadaic acid (OA), a diarrheic shellfish poisoning (DSP) toxin, has been developed, which is a combination of enzyme inhibition assay and flow injection analysis (FIA) utilizing immobilized pyruvate oxidase (PyOx). The sensor system consists of an oxygen electrode with immobilized PyOx, a recorder, an air pump, a HPLC pump and a buffer tank. The sensor is based on the principle that protein phosphatase 2A (PP2A) is inhibited by OA. OA concentration was determined from the amount of oxygen that corresponded to the amount of phosphate ion released from a phosphoprotein in the presence of PP2A, and that was consumed by PyOx. The optimum conditions for the proposed sensor system were as follows: 20 mM PIPES (pH 7.0) containing 1 mM TPP, 5 mM MgCl2 and 1 mM pyruvic acid at final concentrations, a flow rate of 0.15 ml min–1 and an injection volume of 50 µl. The results obtained by the sensor system and the enzyme-linked immunosorbent assay (ELISA) were compared and agreement was observed at OA concentrations ranging from 0.1 to 4 ppb. The detection limit of the proposed sensor was 0.1 ppb, which was approximately 50 times more sensitive than that of ELISA.