The toxicity of a cyanobacterial strain Cylindrospermopsis raciborskii was evaluated in acute toxicity tests with mouse, Daphnia and fish. The test with mouse was performed with aqueous extracts of lyophilized material. The acute tests with Daphnia were performed with three types of samples: (A) culture of cyanobacteria in ASM-1 medium; (B) concentrated cianobacterial cells in culture Daphnia medium; and (C) culture of the strain sonicated and filtered in membranes (45 μm). As controls for each treatment we used ASM-1 and Daphnia culture medium and culture medium with chlorophytes as food for Daphnia sonicated and filtered in membranes (45 μm), respectively. In the acute toxicity test with fish, only sample A was used. The average LD50 (24h) in the mouse bioassays was 16.0±4.3 mg kg-1 and the symptoms were characteristic of neurotoxicosis. The three types of samples were toxic to Daphnia, with sample A as the most toxic and sample C the less toxic. The fish did not show any symptoms, showing greater resistance than Daphnia to this cyanobacterium. We conclude that, since this strain of C. raciborskii is highly toxic to mouse, when blooms of this cyanobacterium occurs in waters for public supply, special care must be taken to eliminate toxins from the water due to risks to human health. Also, since this strain is highly toxic to Daphnia, zooplankton populations may be affected in the natural environments where this cyanobacterium occurs.