Cypermethrin (CYP) is a synthetic pyrethroid insecticide considered to be environmentally safe and widely used in agriculture and veterinary medicine. To ascertain the validity of this opinion, we investigated the mutagenic potential of CYP using the dominant lethal assay in male Swiss albino mice. CYP was administered by gavage at the dose of 20, 40, and 80 mg/kg body weight, dissolved in 0.2 mL corn oil. Treated mice from all groups were mated with untreated virgin females for a period of 6 weeks that covers the entirespermatogenetic cycle. In the pregnant females, we found a high rate of pre- and post-implantation losses. Dominant lethal mutations were induced in a benzo(a)pyrene-treated group (positive control), and a reduction in the number of total implants was found in all CYP-treated groups only during the initial mating weeks. No significant pre-implantation losses were noted in any of the tested doses. However, significant postimplantation losses were identified in the medium and high doses of CYP. A dose-dependent decline in the number of living implants was noticed in all CYP-treated animals during the first 3 weeks, but decreased in the later weeks. The average mutagenic index of 6 weeks was significantly increased only in the high CYP dose. Our results showed that CYP has mutagenic activity, inducing dominant lethal mutations in male germ cells of mice and caution is recommended in the use of this insecticide.