The Cochliomyia hominivorax (Coquerel) is the main agent of primary myiasis in the South America, generating severe economic losses to livestock industry. In Brazil, its control has been done by the application of insecticides, but the inappropriate use of these chemical agents can select resistant flies. It has been shown in another species of the Calliphoridae family, Lucilia cuprina, that a Gly137 → Asp substitution in the enzyme carboxylesterase (E3) converts its typical activity to an organophosphate (OP) hydrolase activity resulting in the development of OP resistance in this species. Preliminary efforts to isolate and characterize the E3 gene in C. hominivorax using primers previously described for L. cuprina resulted in a gene fragment of ≈ 200 bp containing the site of the Gly137 → Asp mutation. This region showed high similarity to the orthologous region in L. cuprina. The intron position is conserved, however its physical size and nucleotide sequence are not. The partial exon sequence differed from that in L. cuprina at 2 nucleotide sites, but no amino acid substitution was predicted. Further analysis will be conducted to determine the association of this point mutation OP resistance in C. hominivorax strains. This characterization should allow the identification of resistant individuals, enabling more effective control strategies.