Micronucleus (MN) assay has been extensively used in the evaluation of DNA damage. Mutagenesis and genotoxicity studies employed this methodology to evaluate possible genotoxic risk due to exposition to hazardous xenobiotics in different organisms, including aquatic sentinel organisms. MN assay in such species is sensitive, fast, and an important biomarker of mutagenic exposure in the environment. The use of bioassays, considering the toxic effects of isolated or combined contaminants, is also important since the environmental variants are minimized. The aim of this study is to gather and evaluate published data on the use of fish MN assay in biomonitoring and genotoxicity assays. In addition, we show the results of both micronuclei and other nuclear abnormalities in erythrocytes from Colossoma macropomum, exposed to methylmercury. Specimens (n = 9) were subjected to 2 mg/L of methylmercury, with an equal control group. Chi-square test was performed to compare the frequencies of nuclear abnormalities between control and treatment groups. The contingence table of χ 2 test showed a significant increase of altered cells in the exposed group. Our results support the importance of MN test as an effective indicator for genotoxicity in fishes, which can be used with exposition bioindicators of human populations exposed to chemical pollutants of consuming water.