We collected 11 groups of Aedes aegypti mosquitoes from various locations of Thailand. After rearing in the laboratory, the colonies were tested for KT50 values to dl,d-T80-allethrin 0.5% mosquito coils in a 25 m3 room semi-field test and KD50 and LD50 values to dl,d-T80-allethrin by a topical application method. Two groups of mosquitoes were susceptible to allethrin similar to a SS (known allethrin sensitive) group, and other 9 groups showed various levels of lower susceptibility to allethrin; of these 6 had susceptibilities similar to a BS (known allethrin resistant) group with extremely low susceptibility, while the remaining 3 groups had susceptibilities to allethrin between the SS and BS groups. The KD50 values with the topical application were found to correlate highly with the KT50 values in the 25 m3 room semi-field test, providing a useful test method for insect susceptibility evaluation. The allethrin mosquito coils, even at higher concentrations, had no activity against the 6 decreased susceptibility groups, similar to the BS group. With the 25 m3 room semi-field test, mosquito coils with d,d-T-prallethrin at concentrations of 0.1 to 0.15% plus a synergist and those with methoxymethyl-tetrafluorobenzyl tetramethylcyclopropane carboxylate (K-3050) at a concentration of 0.1% plus a synergist were found to be highly effective against these mosquito groups. These two pyrethroids had smaller KD50 and LD50 values for topical application, and were more effective than dl,d-T80-allethrin, having the potential to control Ae. aegypti mosquitoes with low allethrin susceptibility.