During the endemic seasons, volunteers use temephos to kill larvae and deltamethrin is the main synthetic pyrethroid used to control adult Aedes mosquitoes through mass spraying (Swaddiwadhipong et al., 1992). Temephos is an organophosphorus insecticide that has been used as a larvicide against Ae. aegypti in Thailand since 1967 (Jurjevskis and Stiles, 1978). The widespread use of insecticides has led to insecticide resistance in mosquitoes that will be another problem for the ability to control disease (Robert and Andre, 1994). Recently, several cases of field associated resistance have been reported in Ae. aegypti against temephos products. Studies of resistance to temephos in Ae. aegypti are undertaken to obtain information on susceptibility of the insect. The susceptibility condition test has been used for Ae. aegypti larva on field collected larvae from urban areas of northeast provinces where high numbers of dengue hemorrhagic fever cases have been reported. These populations had prior history of exposure to temephos. This study is to characterize the resistance to temephos of Ae. aegypti larvae in seven provinces and to determine whether selection of temephos will result in tolerance or resistance in these populations. Such knowledge is essential in defining future control strategies against this medically important mosquito.