Three distinct malathion carboxylesterase (MCE) phenotypes have been identified among strains of Lucilia cuprina. The high-activity phenotype shows 1.6- and 33-fold more MCE specific activity than the intermediate- and low-activity phenotypes, respectively. Flies with high MCE activity are 1000-fold more resistant to malathion than flies with either low or intermediate MCE phenotypes, which are equally susceptible. High and low MCE specific activity are allelic and encoded by the Rmal gene on chromosome 4. Rmal is clustered within one map unit of two other esterase genes, Rop1 and E9, which are implicated in resistance to other organophosphate insecticides. Intermediate MCE specific activity is also inherited within the cluster, although its allelism to Rmal, Rop1, or E9 is unclear. The cluster does not contain the gene for the hemolymph esterase E4, which maps 6.1 map units from Rop1, on the other side of the bubbled wing marker. The cluster appears to be homologous to part of a tandem array of 11 esterase genes on chromosome 3R of Drosophila melanogaster.