¹Department of Pharmaceutical Toxicology, Faculty of Pharmacy, Istanbul University, 34116 Istanbul, Turkey DOI : 10.12991/mpj.2017.26 Diclofop-methyl is a selective post-emergence graminicide from the phenoxy propionate group of herbicides to be developed for control of wild oats, millets, and other annual grass weeds. Diclofop-methyl usage is limited in various grass weed species due to its toxic effect and exposure risks. However, total annual usage of is approximately 750.000 pounds in United States, and more in Asia. Therefore, we aimed to investigate diclofopmethyl"s toxic potentials in vitro and the following assays were used; MTT assay for cytotoxicity, comet assay for genotoxicity, generation of reactive oxygen species (ROS), malondialdehyde (MDA) and glutathione (GSH) for the potential of oxidative damage in mouse embryo fibroblast (NIH/3T3) cell line. Diclofop-methyl was observed to reduce the cell viability in a concentration manner and the half maximal inhibitory concentration (IC₅₀) value was 301.7 μM. Diclofop-methyl caused DNA damage and oxidative stress at the concentrations between 12.5-400 μM. Tail intensities were at the range of 1.24- 58.21% with increasing concentrations, which are approximately ≤ 1.63-fold of the negative control. Also, MDA levels were increased ≥ ³11.4-fold of the negative control that denotes lipid peroxidation was induced. However, there was no significant increment in the ROS and GSH levels at all concentrations. In view of the fact that ROS has not been detected, despite its level of MDA proffers, the idea that oxidative damage may have been caused by other mechanisms. Our results indicate that diclofopmethyl was cytotoxic, genotoxic and might have oxidative damage potential in vitro conditions. Keywords : Diclofop-methyl; reactive oxygen species; malondialdehyde; Comet assay