The issues presented in this study concern the important problem of the occurrence of cyanobacterial blooms in surface water used for water supply purposes. The objective of this study was to analyze the occurrence of cyanotoxic risk in the catchment area of the Obrzyca River, which is a source of drinking water for the inhabitants of Zielona Góra. At the points of Uście and at its tributary at Wojnowo, the river was more exposed to the blooms of toxigenic algae, whose active microcystins synthesize more here than in other places (Sadowo, Chwalim, Ostrzyce, Lubiatów). Throughout the study, conducted in 2008-12, we observed several times that the amounts of cyanobacteria exceeded 500·10³ individuals per litre (ind.·dm^-3). The observed cyanobacterial blooms were dominated by the following species: Dolichospermum flos-aquae and Planktothrix agardhii in the Obrzyca River at Uście, and D. spiroides, D. affinis, and Aphanizomenon flos-aquae at its tributary in Wojnowo. The maximum value of intracellular MC-LR, totalling 21.4 μg·dm^-3, was found at Wojnowo in September 2011. The analysis of the physico-chemical water quality indicators showed that the places along the Obrzyca River where periodic cyanobacterial blooms took place were characterized by eutrophic conditions, and the total nitrogen to total phosphorus ratio (N/P) fell within the range of 10 to 16, and the water temperature exceeded 20ºC. In order to assess the toxic hazard caused by periodic cyanobacterial blooms in the catchment area of the Obrzyca River for drinking water for Zielona Góra residents, regular monitoring of the cyanobacteria and their toxins is required – especially at the points in Uście and Wojnowo as well as at the water intake at the water treatment plant (WTP) in Zawada (in central-western Poland).