The susceptibility to Brugia malayi infections of F2 and F4 progenies of Aedes aegypti (Black Eye strain) treated with ethyl methanesulfonate (EMS) was tested. Both 3-day-old males and females were treated with 0.025, 0.050, 0.075 and 0.10% EMS. Control and treated females were then mated with normal or treated males to recover F1 progeny. F2 offspring were derived from sibling intercrosses, and 3 lines were established by further intercross matings to generate the F4. Susceptibility in the 0.025, 0.050, 0.075 and 0.10% EMS F2s was reduced by 13, 12, 4 and 25%, respectively. The 0.025% and 0.050% EMS F2 females showed a 29 to 39% decrease in mean L3 numbers. At 0.075 and 0.10% EMS, mean L3 numbers decreased by 0.8 and 71%, respectively. The F4 populations gave overall infections of 65, 56 and 22% for the control, 0.25 and 0.10% EMS lines, respectively. Mean L3s were reduced by 24 and 77%, respectively, in the 0.025 and 0.10% F4 EMS-selected populations.