Deltamethrin is a member of pyretroids with a potent effect on parasites. It is a very effective compound having the general characteristics of this group in a synthetic dibromo pyrethroid structure. Deltamethrin has a wide range of usage particularly in agriculture, preventive medicine programs, farm and pet animals. In the studies on toxicity, results were characterized by cholinergic effects, and no evident chronic toxic effect was observed in 2 years of feeding studies on rats. Although it is reported by EPA and FDA that it has no particular toxic effect, these findings are not in accordance with the results of genotoxicity studies. The result of the genotoxicity studies changes according to the live material used, the application route and dose of the compound. In the present study, genotoxicity of delthamethrin at the doses of 50, 100, 200 mg/kg/bw was investigated by in vivo micronucleus test. Deltamethrin was given to the mice intraperitoneally in 10% tween 80 with the doses presented above. To the positive control group, Mitomycin C, which is a mutagen agent, prepared in saline was given in 2 mg/kg/bw doses. At the 48th h after the application, by collecting bone marrow, blood and spleen samples, the formation of micronuclei in erythrocytes and splenocytes was investigated microscopically. In deltamethrin treatment groups, micronuclei frequency was determined and the results were found statistically significant when compared with negative control group (P < 0.001). The micronuclei numbers obtained from different tissue samples of the same animal were also statistically significant (P < 0.001). Particularly in the highest dose group, results were similar to the positive control group. According to these results, it was determined that in acute toxic dose deltamethrin showed genetic toxicity on somatic cells of mice.