Deltamethrin, a synthetic pyrethroid insecticide, was administered to adult female albino rats as a single i.p., s.c., or oral dose of 5.6, 8.4, or 11.2 mg/kg b.w. or repeated i.p. doses of 2.24 mg/kg b.w. for five consecutive days (cumulative dose 11.2 mg/kg b.w.). This treatment inhibited the mitotic index in a dose-dependent manner and increased the frequency of chromosome aberrations in the bone marrow at 24 h post exposure. The parenterally (i.p. and s.c.) administered deltamethrin appeared more effective than the oral gavage for eliciting its cytotoxicity and genetic toxicity potential. The frequency of micronucleated erythrocytes in the bone marrow was also increased at 30 h following a single i.p. dose of 5.6, 8.4, or 11.2 mg/kg b.w. The most prevalent abnormality observed in this study was endomitotic reduplication of chromosomes which, along with mitotic inhibition and micronucleus induction, indicated microtubular/mitotic spindle poisoning by deltamethrin. The increased frequency of chromosome aberrations and micronucleated erythrocytes also suggests a clastogenic potential of deltamethrin. These observations indicate the in vivo susceptibility of mammals to the genetic toxicity potential of deltamethrin.