AbstractIn this study, toxic effects of the cypermethrin in Allium cepa L. cells were investigated. For this aim, we investigated the changes in pigment contents, antioxidant enzymes, mitotic index and chromosomal abnormalities as indicators of toxicity. The seeds were treated with different doses (1.5, 3.0, 6.0 ppm) of cypermethrin for 72 h. The result showed that there was a significant alteration in the tested parameters depending on treatment dose in the seeds exposed to cypermethrin when compared to the control group. Cypermethrin exposure significantly reduced the carotenoid, chlorophyll a and b pigments in all treatment groups. The activity of superoxide dismutase showed a concentration‐time dependent increase and the maximum increase was observed on day 15 of treatment at 6.0 ppm cypermethrin exposure. The activity of catalase increased gradually with increasing cypermethrin concentration, but a soft decrease in CAT activity was decreased after 15 days of 1.5 ppm and 3.0 ppm cypermethrin treatment. In the roots treated with 1.5, 3.0, and 6.0 ppm cypermethrin, the level of malondialdehyde was about 1.8, 2.4, and 3.4 times higher than the control group, respectively. It was also found that cypermethrin has a mitodepressive action on mitosis, and the MI was decreased depending on the dose of cyprmethrin. All of the concentrations of cypermethrin induced chromosomal abnormalities and the most common abnormality observed in the present study was chromosome bridges. © 2011 Wiley Periodicals, Inc. Environ Toxicol 2012.