A study of the molecular genetics of the horn fly, Haematobia irritans irritans (L.), has resulted in the isolation and sequencing of a 2316-bp complementary DNA (cDNA) clone, HFN2b. An open reading frame was found, the deduced 464 amino acid sequence of which encodes a protein with 40% identity to AFG1, a member of a yeast family of ATPases; it contains an ATP- or GTP-binding P-loop motif. Northern and Southern blot analyses do not reveal significant differences in the expression of the gene homologous to HFN2b when comparing laboratory-reared horn flies that are susceptible or resistant to pyrethroid insecticides.