Up to now, less informations of the localization and the origin of toxins are known from HAB causing organisms. Therefore, several toxic dinoflagellates of the genus Alexandrium (A. lusitanicum, A. tamarense, A. catenella) with different toxin patterns, detected with HPLC, were used. They were stained for PSP toxins with a polyclonal antibody against neoSTX/GTX and a monoclonal antibody against STX to locate the distribution of toxins inside the dinoflagellate cell or in associated bacteria. Fluorescent labeled secondary antibodies were used to detect the signal with laser scanning microscopy. Different to the distribution of okadaic acid we found PSP toxins located near the cell periphery of the cell concentrated in small patches near the plasmamembrane and the amphiesmal vesicles. Our investigations are showing different affinities of the antibodies to the different species of Alexandrium used. Cells of the same culture showing very variable toxin contents probably reflecting their different physiological conditions. Dinoflagellates shading off their thecal plates are showing a labeling at the plasmamembrane of the naked cells and the isolated thecal plates. All cultures we used were not kept under axenic conditions, but cells were not inhabiting intracellular bacteria as we know from our ultrastructural investigations. A localization of the toxins in bacteria present outside the dinoflagellates could not be observed.Supported by the German BMBF, TEPS-Project