Fish are often used as sentinel organisms for ecotoxicologi-cal studies because they play numerous roles in the trophic web,they can toxic substances and respond to low concentration of xenobiotics [7]. Oreochorimis niloticus and Geophagus brasiliensiswere chosen as bioindicators in this study due its easy adaptationto laboratory conditions, wide distribution and easy to obtain, andthey are extensively consumed by humans. Despite being used inthe field over 15 years, there are very few studies that evaluate pos-sible adverse effects of mesotrione in living organisms, and any teston fish using biomarkers was performed. Thus, this study aims toevaluate if this compound may generate oxidative stress and DNAdamage in these two species.