Yessotoxin (YTX) is a disulfated polyether toxin produced by marine dinoflagellates. Although there is no clear evidence that YTX is toxic to humans, it is a major cause of false positives in DSP toxin detection by mouse bioassay. We developed a new detection and quantification method for yessotoxin using a BiaCore X Surface plasmon resonance (SPR)-based biosensor. The assay is based in the interaction of YTX with phosphodiesterase enzymes (PDE), one of its cellular targets. The injection of several YTX concentrations (3-12 microM) over immobilized PDE I, showed a dose dependent binding signal, which K(obs) (observed rate constant) allowed us to obtain a calibration curve with a linear fit. The detection of yessotoxin using SPR-based biosensor allows the quantification of the toxin with an automated and repetitive method at concentrations in the range of the 1 mg kg(-1) European regulatory limit.