SUMMARY.1A description is given of the topography and vegetation of Bwamba County, Uganda, an area where human yellow fever has recently occurred. The county is mainly low‐lying and is extensively forested. It lies in the extreme west of Uganda, and includes part of the edge of the Ituri Forest.2The zoogeography of the mosquito fauna is discussed and it is shown that Bwamba belongs to the faunistic district known as the Lower Guinea Forest.3It is pointed out that the behaviour of well‐known species of mosquitoes in forest areas may differ from that usually considered normal.Aëdes ægyptiandAnopheles gambiæare taken as examples, and evidence is given to show that in Bwamba these species, normally considered to be domestic and homophilic, appear to be largely sylvan and zoo