The main objective of this work was to use micronucleus induction in fish erythrocytes to study the risk to aquatic ecosystems due to the genotoxicity of Chlorotriazine Reactive Azo Red 120 textile dye. The frequencies of micronuclei were studied for three low doses of 1, 5, and 10 mg/L and blood sampling was carried out on the same fish after 3, 6, and 9 days. It was found that micronuclei increased not only in a dose-dependent manner but also in a time-dependent way, compared with negative (tap water) and positive (10 ppm benzene) control groups. There was also a slight, time-dependent increase in erythrocyte micronuclei of the control fish specimens. This study proved the genotoxicity of this dye, and suggests that further studies should be made on other dyes and some other toxic industrial pollutant discharges in water ecosystems, using fish as an indicator to monitor pollutant genotoxicity.