Anisopteromalus calandrae (Howard) (Hymenoptera : Pteromalidae) is an important parasitoid of stored-grain insect pests. Partial cDNA sequences of an esterase-like enzyme have been obtained from a malathion-resistant (R) strain and a susceptible (S) strain of this wasp. A single-base substitution in the R strain has been conürmed by using PCR ampliücation of speciüc allele (PASA) to amplify genomic DNA extracted from individual resistant and susceptible parents, hybrids from F 1 double reciprocal crosses, and progeny from backcrosses. The R allele appeared to be inherited in a strict Mendelian fashion in both diploid female and haploid male progeny. The esterase fragment co-segregated with resistance in these crosses and backcrosses. Female wasps in a mixed population of A calandrae that survived a malathion screen carried the R allele for the esterase-like enzyme, while those wasps that died did not have the R allele. The single base-pair mutation, guanine in the R strain and thymine in the S strain, presumably results in a tryptophan-to-glycine amino acid substitution in the encoded protein. We do not know how these amino acid substitutions may relate to functional diþ erences in the enzyme. However, this esterase gene or another linked esterase gene may encode the resistance-associated malathion detoxifying activity in the R strain.