Abstract: The effects of microcystin produced by the toxic cyanobacterial strain Microcystis aeruginosa on bioaccumulation and antioxidant enzymes (superoxide dismutase and catalase) of tilapia fish (collected from Al-Khadoud spring, Al-Hasa, Saudi Arabia) were investigated. The results showed that microcystin contained in cyanobacterial blooms induce CAT and SOD activity in a time-dependent manner. The data also shows that microcystin concentration in muscle was much lower than in liver, as the liver is the target organ of these toxins. In addition, microcystin concentration in faeces increased gradually until the end of the treatment period.