The genotoxic potential of the pyrethroid flumethrin was evaluated by using the combined protocol of metaphase analysis and micronucleus test in vivo in mouse bone marrow. The dermal route was tested in a single treatment and the intraperitoneal (i.p.) route in a single and a multiple treatment. Flumethrin showed a cytotoxic effect on both myelopoiesis and erythropoiesis, as evidenced by a reduction in the mitotic index and in polychromatic erythrocyte values. An increase in the frequency of gaps after the dermal exposure and of breaks only at the highest dose tested in the i.p. treatment indicates a weak clastogenic potential of the compound. A significant increase in the frequency of micronucleated polychromatic erythrocytes was observed after single and multiple i.p. treatments. In the latter, the induction of micronuclei was highly significant but not accompanied by an increase in breaks. This may indicate that the clastogenic effect might not account by itself for the induction of micronuclei, which could also have arisen from an aneugenic potential of flumethrin.