Physicochemical and insecticidal properties of fenitrothion microcapsule (MC) as a new residual spraying formulation for mosquito control were studied. The wall staining after treatment with MC at the rate of 1-2 g AI/m2 was not noticeable as compared with that of wettable powder (WP). Covering rate with MC on a vertical wall was almost the same as WP and emulsifiable concentrate (EC) when the wall material was absorptive plywood and mud, and was relatively higher than WP and EC when the wall material was nonabsorptive overlaid plywood. Residual efficacy of MC at the rate of 1 g/m2 was almost equivalent or superior to that of WP at the rate of 2 g/m2. Fenitrothion MC also has had an "airborne" effect, which is sort of a vapor killing effect of insecticides under high vapor pressure. This suggested a difference in mode of action in mosquitoes from that for cockroach control.