A DETAILED study of the nature and mode of inheritance of dieldrin-resistance in a strain of Anopheles gambiae from the Western Sokoto region of Northern Nigeria, and of DDT-resistance in a strain of A. sundaicus from Indonesia has already been made and recorded1–3. Dieldrin-resistance in A. gambiae was shown to depend on a single genetic factor semi-dominant in its manifestation and imparting cross-resistance to other cyclodiene chlorinated hydrocarbons and to γ-BHC but not to DDT. DDT-resistance in A. sundaicus also shows a mono-factorial type of inheritance, but here the factor is recessive in nature and imparts cross-resistance to DDT-analogues but not to dieldrin or to γ-BHC.