A fast fluorimetric assay (FFA) for the detection of saxitoxin and its congeners in plankton samples is presented and is compared with the analysis by HPLC. The correlation between the results of the assay and those of the HPLC is significant for most of the carbamoyl saxitoxins. In culture experiments, saxitoxin producing dinoflagellates could be clearly distinguished from those which do not produce this toxin by use of FFA. During research cruises to the Orkney Islands and the Firth of Forth (Scotland) in 1997 and 1998, samples of toxic Alexandrium tamarense blooms were analysed by FFA. In 1998 detailed vertical profiles from the surface to 40 m water depth were sampled during two drifting experiments east of the Firth of Forth and east of the Orkney islands. We were able to show that saxitoxin fluorescence was found predominantly in the upper water layer and was not correlated with chlorophyll a concentration as measured by the Chlorophyll sensor of the CTD probe. The fast fluorimetric assay is easy to use and can be a helpful tool for ecological and also monitoring tasks concerning the detection of saxitoxin producing dinoflagellates.