Toxic cyanobacteria in public water supply reservoirs represent a serious health risk as they can release potent cyanotoxins into the water. In the present study we analyzed surface water collected from the Utinga Reservoir, the main source of drinking water for the city of Belem-PA, in order to characterize the reservoir's cyanobacterial biota and to determine the toxicity of these organisms and the concentration of microcystins in raw and treated water. The cyanobacterial biota included potentially toxic genera such as Aphanizomenon, Microcystis, Nostoc, Oscillatoria, Planktothrix and Radiocystis. Mouse bioassays revealed a 43.6% frequency of hepatotoxic strains, including 30.8% Microcystis viridis strains and 12.8% Radiocystis fernandoi strains, with the lowest LD(100) (ip) of 45 and 75 mg kg(-1) body weight, respectively. Subacute hepatotoxicity was observed for Aphanizomenon cf. gracile and Coelomoron pusillum strains. HPLC analysis confirmed the production of microcystins at maximum concentrations of 4.22 microg mg(-1) dry weight for M. viridis and 2.47 microg mg(-1) for R. fernandoi. Microcystins at concentrations of up to 1.25 microg L(-1) detected in raw water by ELISA, together with a cyanobacterial density of 20,000 cells mL(-1), represents the first report of a bloom of cyanobacteria for an Amazonian water body.