A fast fluorimetric assay (FFA) for the detection of saxitoxins in plankton samples was compared with the analysis by high performance liquid chromatography (HPLC). The correlation between the results of the assay and those of the HPLC is significant for most of the carbamoyl saxitoxins. During research cruises to the Orkney Islands and the Firth of Forth (Scotland) in 1997 and 1998, samples of toxic Alexandrium spp. blooms were analyzed successfully by FFA and HPLC. Because the toxin pattern of toxic Alexandrium spp. mostly consists of highly fluorescent saxitoxins (like STX and GTX3), the assay can be applied for monitoring toxic Alexandrium spp. blooms and is suggested as a tool for future prewarning systems of paralytic shellfish poisoning (PSP) events.