Organophosphorous pesticides (OPs) posses a great potential of acute toxicity for exposed animals and men. To evaluate the toxic potential of the organophosphate diazinon on root meristematic cells of Allium cepa L., was created two groups: In group 1 (control group), was not given any chemical. In group 2 (diazinon-treatment group), different doses (10, 40, 80 and 160 ppm) and times periods (24, 48 and 72 h) were administered. After exposure, cell death, effective concentration (EC50), mitotic index, cellular /chromosome aberrations, DNA damage by comet assay and RAPD-PCR were assessed at exposure times. EC50 value of diazinon was detected approximately 80 ppm. Hyperchromasia, later segragation, micronucleus, pulverised nucleus, nuclear cytoplasmic shrinkage and cell death, cytoplasmic vacuolation were detected in meristem cells as chromosome/celular aberrations for 72 h at 80 ppm. DNA damage was identified using tail DNA%, tail lengths and tail moment from these cells. Increasing exposure doses of diazinon caused increasing tail DNA% and tail lengths at 72 h. DNA bands of increasing concentrations treated groups were more distant to compare with the control group according to RAPD-PCR method. Diazinon cause cytotoxic and genotoxic on A. cepa root and could be considered for further toxicological evaluations.