Phytoplankton samples were harvested off the south-west coast of Ireland using a large composite plankton net (590 x 120 cm) with an outer net of 50 µm and an inner net of 108 µm mesh sizes which was effective for collecting biomass rich in Dinophysis sp. A new rapid, sensitive HPLC method has been developed for the determination of acidic diarrhetic shellfish poisoning (DSP) toxins and this has been applied to the determination of toxin profiles in phytoplankton. Extracts from phytoplankton samples were reacted with 3-bromomethyl-6,7-dimethoxy-1-methyl-2(1)-quinoxalinone (BrDMEQ) and 5% diisopropylethylamine at 50°C for 20 min. Sample cleanup was achieved using silica solid phase extraction (SPE) with a 83 ± 2% recovery for okadaic acid (OA) derivatives. Isocratic reversed phase HPLC was carried out using acetonitrile/water (57/43) with fluorimetric detection. Calibrations were linear (R2 = 0.991) in the typical analytical range of 1 - 10 ng OA equivalents injected. OA, DTX-2 and pectenotoxin-2 seco acids (PTX2SA) were determined in phytoplankton as their DMEQ derivatives and toxin identification was confirmed using LC-MS and LC-MSn. In a comparative study with the reagent, 9-anthryldiazomethane (ADAM), which was applied to the analysis of toxins in phytoplankton, the BrDMEQ method was not only more sensitive than the ADAM method but improved resolution of the polyether toxins derivatives was achieved. Although, both OA and DTX-2 were found in all phytoplankton samples that contained Dinophysis sp., PTX2SA compounds were detected in only a small number of samples.