Annual rain deficit and lack of water renewal in 1998 linked to El NiÒo consequences seem to be responsible for the toxic cyanobacterial blooms of many water supply reservoirs of the Brazilian northeast. In Pernambuco State, there are growing records of neurotoxic Cylindrospermopsis blooms. The Tapacur· reservoir provides water for a population of 1 million inhabitants of the city of Recife, with total volume capacity of 94,200,000 m3. It is an hypereuthophic reservoir and cyanobacterial blooms are frequent around the year. This reservoir has been studied since May 1998 and the following variables were measured: cyanobacterial toxins by mouse bioassay, HPLC and imunoenzymatic assay (ELISA); phytoplankton population densities and composition according to Utermohl method, and limnological variables by standard techniques. Cyanobacteria were the dominant group of algae, representing at least 88% of the phytoplanktonic community. Cyanobacterial densities as high as 84,332 org/mL and chlorophyll values of 45.6 µg/L were observed on July 1998. At this period, Cylindrospermopsis raciborskii was the dominant species. On May 1999 a chlorophyll peak value of 88.7 µg/L was observed before the rainy season. Bloom extracts were tested by mouse bioassay and the symptoms observed were tremors, convulsion and respiratory arrest, with the time of animal death ranging from 2 to 5 minutes. Bloom samples collected from September 1998 to June 1999 showed constant neurotoxicity. A preliminary analysis indicated the presence of Paralytic Shellfish Poisons (PSP). On extracts of shrimp (Macrobractum amazonicum) collected from the reservoir, this analysis also indicated the presence of 24 µg STXeq/100g. Microcystin concentrations on phytoplanktonic cells were found to be low (mean value 0.44 ng/L). Despite of the presence of PSP toxins, Tapacur· waters continued to be used for supplying the inhabitants of Recife.