A strain of Anopheles culicifaciesGiles (species B) originally collected from Aurangabad, Maharashtra State, India, showed highly specific resistance to malathion. Homogenates of one-day-old adults metabolized 14C-labelled malathion at a level comparable with strains of A. stephensi Liston and A. arabiensis Patton known to possess the malathion carboxylesterase resistance mechanism and at a higher rate (6.4 and 7.8 times, respectively) than malathion-susceptible strains of these species. The major metabolite obtained was malathion dicarboxylic acid, a carboxylesterase product, which suggests that this may be the principal resistance mechanism in this strain. Levels of non-specific esterase activity in larvae and adults were similar to those shown by both the susceptible and resistant strains of A. stephensi, implying that no quantitative increase in esterases had occurred. Microtitre plate assays on individual mosquitoes provided no evidence for the presence of the insensitive acetylcholinesterase resistance mechanism.