As insecticide resistance may jeopardize the successful malaria control programmes in the Mekong region， a large investigation was previously conducted in the Mekong countries to assess the susceptibility of the main malaria vectors against DDT and pyrethroid insecticides. It showed that the main vector， Anopheles epiroticus， was highly pyrethroid-resistant in the Mekong delta， whereas Anopheles minimus sensu lato was pyrethroid-resistant in northern Vietnam. Anopheles dirus sensu stricto showed possible resistance to type II pyrethroids in central Vietnam. Anopheles subpictus was DDT- and pyrethroid-resistant in the Mekong Delta. The present study intends to explore the resistance mechanisms involved.