Thiacloprid, a neonicotinoid insecticide, is widely used for controlling various species of pests on many crops. The potential genotoxic effects of thiacloprid on human peripheral blood lymphocytes (PBLs) were investigated in vitro by the chromosome aberrations (CAs), sister chromatid exchanges (SCEs), and cytokinesis-block micronucleus (MN) assays. The human PBLs were treated with 75, 150, and 300 mu g/mL thiacloprid in the absence and presence of an exogenous metabolic activator (S9 mix). Thiacloprid increased the CAs and SCEs significantly at all concentrations (75, 150, and 300 mu g/mL) both in the absence and presence of the S9 mix and induced a significant increase in MN and nucleoplasmic bridge formations at all concentrations for 24 h and at 75 and 150 mu g/mL for 48-h treatment periods in the absence of the S9 mix; and at all concentrations in the presence of the S9 mix when compared with the control and solvent control. Thiacloprid was also found to significantly induce nuclear bud (NBUD) formation at 300 mu g/mL for 24 h and at 150 mu g/mL for 48-h treatment times in the absence of the S9 mix and at the two highest concentrations (150 and 300 mu g/mL) in the presence of the S9 mix. Thiacloprid significantly decreased the mitotic index, proliferation index, and nuclear division index for all concentrations both in the absence and presence of the S9 mix. (c) 2012 Wiley Periodicals, Inc. Environ Toxicol 29: 631-641, 2014.