The genotoxicity of cypermethrin, a synthetic pyrethroid insecticide, has been studied in vivo in mice. Bone marrow chromosome aberrations were not dose, time or route dependent, but most of the results differed significantly from controls except after 6 and 24 h of treatment using the i.p. and s.c. routes, respectively. In the micronucleus test, the occurrence of polychromatic erythrocytes with micronuclei increased slightly with dose, but was significantly higher than controls at all dose levels. Only a marginal difference in the incidence of sperm abnormalities was noted with the three doses of cypermethrin tested. However, all results differed significantly from the respective control values.