Insecticidal properties of fenitrothion microcapsules (MC3) as a new residual spraying formulation for mosquito control were studied. Adult female Anopheles albimanus were confined so as to contact several surfaces treated with fenitrothion MC or wettable powder (WP) for 10 and 30 min. Residual efficacy of MC at the rate of 1 g/m2 was almost equivalent to that of WP at the rate of 2 g/m2 on a plywood surface and was superior to that on an unglazed pottery surface. Greater than 50% mortality was maintained for more than 28 wk after treatment by fenitrothion MC on a mud surface with 10 min of contact at the rate of 2 g/m2. The effective duration on a mud surface treated with WP was less than 44 wk with 30 min of contact and was less than 12 wk with 10 min of contact at the rate of 2 g/m2. Microcapsule particles were observed to be mechanically broken (trampled) by mosquito contact, and the amount of fenitrothion released from MC increased as contact time increased. Comparison of MC with WP and emulsifiable concentrate formulations in this study indicates that the increased residual activity results from protection from evaporation and decomposition afforded by capsule walls of the MC formulation.