Health risks generated by cyanobacterial toxins in drinking and recreational water were clearly confirmed. During the survey programme (carried out since 1994) on distribution of toxic freshwater cyanobacteria in various water bodies including lake-reservoirs, ponds and rivers of Morocco, many cyanobacteria species dominated by Microcytis genus have been identified. Particular interest has been given to Nostoc species that was found dominant in Oukaïmeden River localised at 2600 m of altitude in high Atlas mountains of Marrakech. This benthic cyanobacteria appears covering the surface of rocks and sediment in a shallow zone of the river particularly along the river’s banks where the current speed was less. The massive growth of Nostoc sp yearly occurred during the period June-September, where the average of water temperature was higher than 15°C. It seems that, an important input of organic matter in the river and particular water physico-chemical conditions were in favours of Nostoc blooms. The toxicity of the bloom material was confirmed both by mousse bioassay and ELISA. The major signs of mice poisoning were severe diarrhea, which appears 15 min after injection. The histopathological studies show a slight liver damage and remarkable destruction of intestinal mucosa consecutive of a collapse of villi architecture and a shedding of enterocyte cells. The detection and the content of total hepatotoxic microcystins was determined by using the Enzyme-linked immunosorbent assay (ELISA) which gave an amount of 12.8µg g ˆ1. Relationship between the content of microcystins and the growth stage of Nostoc were determined and discussed. The toxicity reported for this particular cyanobacterium strain was due to presence of hepatotoxic peptide toxins (microcystins) mixed with others unidentified toxins (likely neurotoxic compounds). The determination of toxigenic profile for this strain was not achieved. These preliminary results revealed potent sanitary risks generated by the existence of these toxic cyanobacteria water blooms in this important tourist site (High Atlas Mountains of Marrakesh region).