The New World screwworm, Cochliomyia hominivorax (Coquerel) (Diptera: Calliphoridae), is one of the most important myiasis-causing flies in South America. It is responsible for severe economic losses to livestock producers, mainly because it causes mortality in newborn calves and reductions in the quality of leather and in the production of milk and meat. The economic losses caused by myiasis, along with those caused by other internal and external parasites, are the main factors limiting meat production. In Brazil, C. hominivorax has been controlled by applying insecticides, particularly organophosphate (OP)-based compounds. However, the improper and continuous use of these chemicals can lead to the selection of OP-resistant strains. This, associated with the fast development of OP resistance in other myiasis-causing flies, shows the importance of investigating resistance in C. hominivorax. Based on the findings of previous studies, the objective of the current work was to isolate and sequence the E3 gene in C. hominivorax. Mutations at the positions (Gly137 and Trp251) responsible for conferring OP resistance in Lucilia cuprina and Musca domestica L. (Muscidae) were identified in C. hominivorax. In addition, the orthologous region in C. hominivorax contained motifs that are highly conserved among carboxyl/cholinesterases and contribute to the catalytic mechanism of the active site. The characterization of this gene in natural populations of New World screwworm can be an important tool for monitoring resistance to insecticides throughout its current geographic distribution. This will provide information for the selection and implementation of more effective pest management programmes. © 2009 The Royal Entomological Society.23SUPPL. 18691Altschul, S.F., Maden, T.L., Schäffer, A.A., Zhang, Z., Miller, W., Lipman, D.J., Gapped BLAST and PSI-BLAST: A new generation of protein database search programs (1997) Nucleic Acids Research, 25, pp. 3389-3402Batterham, P., Davies, A.G., Game, A.Y., McKenzie, J.A., Asymmetry - Where evolutionary and developmental genetics meet (1996) Bioessays, 18, pp. 841-845Campbell, P.M., Newcomb, R.D., Russel, R.J., Oakeshott, J.G., Two different amino acid substitutions in the ali-esterase E3 confer alternative types of organophosphorus insecticide resistance in the sheep blowfly, Lucilia cuprina (1998) Insect Biochemistry and Molecular Biology, 28, pp. 139-150Carvalho, R.A., Torres, T.T., Azeredo-Espin, M.L., A survey of mutations in the Cochliomyia hominivorax (Diptera: Calliphoridae) esterase E3 gene associated with organophosphate resistance and the molecular identification of mutant alleles (2006) Veterinary Parasitology, 140, pp. 344-351Claudianos, C., Russel, R.J., Oakeshott, J.G., The same amino acid substitution in orthologous esterases confers organophosphate resistance on the housefly and a blowfly (1999) Insect Biochemistry and Molecular Biology, 29, pp. 675-686Cygler, M., Schrag, J.D., Sussman, J.L., Harel, M., Silman, I., Gentry, M.K., Doctor, B.P., Relationship between sequence conservation and three-dimensional structure in a large family of esterases, lipases and related proteins (1993) Protein Science, 2, pp. 366-382Ffrench-Constant, R.H., Which came first: Insecticides or resistance? (2007) Trends in Genetics, 23, pp. 1-4Georghiou, G.P., (1983) Management of Resistance in Arthropods. Pest Resistance to Pesticides, pp. 769-792. , ed. by. G. P. Georghiou. T. Saito. pp. Plenum. New YorkGilleard, J.S., Woods, D.J., Dow, J.A., Model organism genomics in veterinary parasite drug discovery (2005) Trends in Parasitology, 21, pp. 302-305Hall, T.A., BioEdit: A user-friendly biological sequence alignment editor and analysis program for Windows 95/98/NT (1999) Nucleic Acids Symposium Series, 41, pp. 95-98Hall, M., Wall, R., Myiasis of human and domestic animals (1995) Advances in Parasitology, 35, pp. 257-334Harel, M., Kryger, G., Rosenberry, T.L., Al, E., Three-dimensional structures of Drosophila melanogaster acetylcholinesterase and of its complexes with two potent inhibitors (2000) Protein Science, 9, pp. 1063-1072Hartley, C.J., Newcomb, R.D., Russel, R.J., Al, E., Amplification of DNA from preserved specimens shows blowflies were pre-adapted for the rapid evolution of insecticide resistance (2006) Proceedings of the National Academy of Sciences of the United States America, 103, pp. 8757-8762Heidari, R., Devonshire, A.L., Campbell, B.E., Bell, K.L., Dorrian, S.J., Oakeshott, J.G., Russell, R.J., Hydrolysis of organophosphorus insecticides by in vitro modified carboxylesterase E3 from Lucilia cuprina (2004) Insect Biochemistry and Molecular Biology, 34, pp. 353-363Heidari, R., Devonshire, A.L., Campbell, B.E., Bell, K.L., Dorrian, S.J., Oakeshott, J.G., Russel, R.J., Hydrolysis of pyrethroids by carboxylesterases from Lucilia cuprina and Drosophila melanogaster with active sites modified by in vitro mutagenesis (2005) Insect Biochemistry and Molecular Biology, 35, pp. 597-609Hemingway, J., Hawkes, N.J., McCarroll, L., Ranson, H., The molecular basis of insecticide resistance in mosquitoes (2004) Insect Biochemistry and Molecular Biology, 34, pp. 653-665(2005) Produção da Pecuária Municipal., 33. , Instituto Brasileiro de Geografia e Estatística (IBGE). IBGE, Rio de Jeneiro. BrazilLevot, G.W., Resistance and the control of sheep ectoparasites (1995) International Journal for Parasitology, 25, pp. 1355-1362Li, X., Schuler, M.A., Berenbaum, M.R., Molecular mechanisms of metabolic resistance to synthetic and natural xenobiotics (2007) Annual Review of Entomology, 52, pp. 231-253Newcomb, R.D., Campbell, P.M., Russel, R.J., Oakeshott, J.G., CDNA cloning, baculovirus expression and kinetic properties of the esterase E3 involved in organophosphate resistance in Lucilia cuprina (1997) Insect Biochemistry and Molecular Biology, 27, pp. 15-25Newcomb, R.D., Campbell, P.M., Ollis, D.L., Cheah, E., Russel, R.J., Oaheshott, J.G., A single amino acid substitution converts a carboxylesterase to an organophosphate hydrolase and confers insecticide resistance on a blowfly (1997) Proceedings of the National Academy of Sciences of the United States America, 94, pp. 7464-7468Newcomb, R.D., Gleeson, D.M., Yong, C.G., Russell, R.J., Oakeshott, J.G., Multiple mutations and gene duplications conferring organophosphorus insecticide resistance have been selected at the Rop-1 locus of the sheep blowfly, Lucilia cuprina (2005) Journal of Molecular Evolution, 60, pp. 207-220Oakeshott, J.G., Devonshire, A.L., Claudianos, C., Al, E., Comparing the organophosphorus and carbamate insecticide resistance mutations in cholin- and carboxyl-esterases (2005) Chemico-Biological Interactions, 157-158, pp. 269-75Roush, R.T., McKenzie, J.A., Ecological genetics of insecticide and acaricide resistance (1987) Annual Review of Entomology, 32, pp. 361-380Sambrook, J., Fritsch, E.F., Maniatis, T., (1989) Molecular Cloning: A Laboratory Manual, 13, pp. 121-132. , pp. Cold Spring Harbor Laboratory Press. New YorkThompson, J.D., Gibson, T.J., Plewniak, F., Jeanmougin, F., Higgins, D.G., The ClustalX Windows interface: Flexible strategies for multiple sequence alignment aided by quality analysis tools (1997) Nucleic Acids Research, 24, pp. 4876-4882(1957) The Insecticide-resistance Problem. Expert Committee on Insecticides., , World Health Organization. 7th Report. WHO Technical Report Service. WHO, GenevaWyss, J.H., Screwworm eradication in the Americas (2000) Annals of the New York Academy of Science, 916, pp. 186-193Zumpt, F., (1965) Myiasis in Man and Animals in the Old World, pp. 1-267. , pp. Butterworths. London