(1) During studies on the mosquito breeding occurring in tree-holes in the vicinity of Lagos, Nigeria, water was collected from holes in 260 trees. This number included 36 species, consisting largely of fruit trees and those used for shade and ornamental purposes.(2) Mosquito larvae were present in samples of water taken from 227, or 87·3 per cent., of the trees.(3) The number of adult mosquitos bred out from all larvae collected totalled 12,285, with 14 species represented.(4) Among the several species, Aëdes aegypti stood second in the number of trees in which it was found and fourth in the number of adults bred out.(5) Aëdes aegypti larvae were taken from holes in trees at a distance of 350 yards from the nearest habitation.(6) The amount of breeding of Aëdes aegypti found occurring in tree-holes was sufficient to demonstrate their importance as a source of mosquitos of this species and the necessity for their elimination as a measure in yellow fever control.