Observations made in Kigezi District, Uganda, had shown a great reduction in the number of Anopheles gambiae entering an experimental hut treated with DDT. The work reported in this paper confirms the phenomenon of reduced entry by A. gambiae and A. funestus in two experiments carried out in Masaka, another district of Uganda, using mud-walled huts, roofed with thatch or corrugated-iron sheets and sprayed with DDT and dieldrin. The fact that no similar reduction was observed with Mansonia (mansonioides) uniformis, a common species in the area, indicates the need to determine and take into account any reduction in the number of entering mosquitos when assessing the effect of residual insecticides. Of interest in these experiments was the finding that DDT and dieldrin produced satisfactory kills with all the local anopheline species in spite of their rapid sorption by the mud walls, an indication of the importance of thatch or metal roofs as a source of active insecticide.