The pyrethroid insecticide cypermethrin was tested for the induction of genetic damage in male germ cells of Drosophila melanogaster. Sex-linked recessive lethals, sex-chromosome loss and non-disjunction were studied following different routes of administration: adult feeding, injection and larval feeding. Our results show that, after adult injection and larval ingest ion, cypermethrin induces a small but significant increase in the frequency of sex-linked recessive lethal mutations. However, no significant increases were observed in the frequency of sex chromosome loss or non-disjunction after exposure of male flies to cypermethrin at concentrations up to 20 p.p.m. © 1986 IRL Press Ltd.