Two aerial applications, 6 days apart, of 50% malathion emulsifiable concentrate were made from a single-engine biplane against Aedes africanus and other sylvatic vectors of yellow fever (YF) in a densely wooded forest habitat in West Africa. The trial demonstrated that aerial application, at the rate of 1 1/ha, can rapidly suppress populations of the sylvatic vectors of YF for a period of time sufficient to interrupt disease transmission Ae. africanus populations did not recover in the forest around Akpugo village until day 13 after the 2nd application of insecticide and were suppressed until day 35 in compounds (indoors and outdoors). For 2 weeks after the malathion applications no mosquito breeding took place in any of the peri-domestic water containers surveyed. Parity rates showed that there was minimal infiltration from outside the trial area of 200 ha for 20 days. The cost of aerial application is much higher than that of ground application, but it is the only means of successfully treating the inaccessible forest areas of south-eastern Nigeria where there are numerous rural communities and yellow fever outbreaks occur from time to time.