Flumetralin, a synthetic plant growth regulator with herbicidal activity belonging to the 2,6-dinitroaniline class of chemicals, has been evaluated for its ability to induce genotoxicity in human peripheral blood lymphocytes (PBLs). The potential genotoxic and cytotoxic effects of flumetralin were investigated in vitro by chromosome aberration (CA) and cytokinesis-block micronucleus assays. Human PBLs were treated with 125, 250, 500, and 1000 µg/mL flumetralin for 24 and 48 h. Flumetralin statistically significantly increased the frequency of structural CAs at the three highest concentrations (250, 500, and 1000 µg/mL) for both treatment periods (24 and 48 h) when compared with both the negative and solvent controls. In addition, micronucleus formation was significantly induced at higher concentrations (250, 500, and 1000 µg/mL) for 24 h and at 125 and 500 µg/mL of flumetralin for the 48-h treatment period compared with the controls. Because of the excessive cytostatic effects of flumetralin, binuclear cells could not be detected sufficiently at the highest two concentrations (500 and 1000 µg/mL) for the 48-h treatment period. Furthermore, flumetralin significantly decreased the mitotic index and nuclear division index for all concentrations and treatment times compared with the control groups. The present results indicate that flumetralin was clastogenic and cytotoxic/cytostatic to human PBLs. This study presents the first report of the genotoxic and cytotoxic properties of flumetralin.