Although thallium is detrimental to all living organisms, information regarding the mutagenic and genotoxic effects of this element and its compounds remains scarce. Therefore, we tested the genotoxic and cytotoxic effects of thallium(I) acetate on human peripheral blood cells in vitro using structural chromosomal aberrations (SCAs), sister chromatid exchanges (SCEs), and single-cell gel electrophoresis (at pH >13 or 12.1) analysis. Whole blood samples were incubated with 0.5, 1, 5, 10, 50, or 100 µg/mL thallium salt. Exposure to this metal compound resulted in a clear dose-dependent reduction in the mitotic and replicative indices. An increase in SCAs was evident in the treated group compared with the control group, and significant differences were observed in the percentage of cells with SCAs when metaphase cells were treated with 0.5–10 µg/mL of thallium(I). The SCE test did not reveal any significant differences. We observed that a 1-h treatment with thallium(I) at pH > 13 significantly increased the comet length for all the concentrations tested; however, at pH 12.1, only the two highest concentrations affected the comet length. These results suggested that thallium(I) acetate induces cytotoxic, cytostatic, and clastogenic effects, as well as DNA damage. © 2013 Wiley Periodicals, Inc. Environ Toxicol 30: 572–580, 2015.