Abstract.The enzymes acetylcholinesterase, glutathione S‐transferase (GST), glucose 6‐phosphate dehydrogenase (G6PD), and general esterases were assayed in four strains ofAedes aegyptimosquitoes aged between 1 and 30 days. Microtitre plate methods were used to assay activity in the homogenates of individual mosquitoes. The levels of GST and G6PD declined with the age of the mosquitoes, while the activity for the other enzymes remained constant. Soluble protein content was also found to decline with mosquito age in all the strains.Insecticide bioassays showed that two strains (Trinidad and Virtudes) ofAe. aegyptiwere resistant to DDT, deltamethrin and malathion, whereas two other strains (Bangkok and Indian) were susceptible to all four classes of insecticides tested.Higher esterase activity levels in the resistant compared to the susceptible strains were assumed to be the cause of organophosphate resistance. The combination of DDT and deltamethrin resistance in two strains with normal GST and G6PD characteristics suggests that a kdr‐type nerve insensitivity mechanism may be inv