Organophosphorus insecticides are used worldwide in the control of agricultural, household and veterinary pests. Dichlorvos(2,2-dichlorovinyl dimethyl phosphate) is a commonly used organophosphorus insecticide. In this study, in order to elucidate the toxic nuclear effects of dichlorvos, freshly isolated human peripheral blood lymphocytes were incubated with 5, 10, 20, 40, 80 and 100 mu g/mL of dichlorvos. According to the results, dichlorvos induced micronuclei, decreased the mitotic and replication indexes. It is a genotoxic product causing chromosomal damage (an increase in micronucleus) and cell death (decrease in mitotic and replication indexes).