Ground applications of malathion thermal fogs and cold mists were made in 2 rural communities where a few species of yellow fever vectors were co-existing. Malathion thermal fogs (5%) dispersed efficiently in dense vegetation and gave a reduction of more than 90% in the crepuscular landing rate of Aedes africanus. However, because the fogs had no residual effect, the population returned to pretreatment levels within 5 days. In contrast, both population reduction and recovery were considerably slower after the 1st malathion cold mist (50% emulsifiable concentrate) application. The 2 successive cold mist applications in a larger community resulted in an immediate and sharp decline (over 95%) in numbers of adult mosquitoes, followed by a 100% reduction in oviposition which lasted for 2 weeks. The population recovery after sequential treatment of malathion cold mists was rather slower in a larger area than in small community, mainly due to low infiltration of mosquitoes from nearby untreated forest. Cold mists penetrated 40 m into forest from foot-paths where spraying was carried out, more droplets fell in trees at a height of 7 m than on the ground, and more droplets settled naturally than by the effect of blasts from the spray machine. These trials show that for communities with dense vegetation and undulating terrain ground application of undiluted insecticides using a knapsack Fontan R12 machine is suitable, and that the Fontan R12 is more suitable than the Swingfog machine for areas larger than 15-20 ha. The technique can be used efficiently by locally recruited personnel to suppress the local yellow fever vector population in forested villages with the aim of interrupting virus transmission.