This study aimed to evaluate the effects of aqueous extracts of dried Phyllanthus niruri L. (stonebreaker) leaves on Allium cepa L. root meristem cells at four concentrations, 0.02 (usual concentration), 0.04, 0.06 and 0.08mg/mL and exposure times of 24 and 48 hours. For each concentration we used a group of five onion bulbs that were first embedded in distilled water and then transferred to their respective concentrations. The radicles were collected and fixed in acetic acid (3:1) for 24 hours. The slides were prepared by the crushing technique and stained with 2% acetic orcein. Cells were analyzed throughout the cell cycle, totaling 5000 for each control and exposure time. The calculated mitotic indices were subjected to the Chi-squared statistical analysis (p<0.05). From the results obtained it was observed that all four concentrations tested had significant antiproliferative effect on the cell cycle of this test system. We also found the presence of cellular aberrations such as colchicined metaphases, anaphasic and telophasic bridges, and micronuclei in the two exposure times for all concentrations evaluated. Therefore, under the conditions studied the concentrations of aqueous extracts of leaves of P. niruri showed to be cytotoxic and genotoxic.