Susceptibility tests were performed in different localities of Northern Iran, which had been treated with DDT for malaria control inside houses, 6-13 rounds once a year, and for agricultural pest control on cotton plantations. These tests showed that in localities where the application of DDT for malaria control has been withdrawn since 1968, the application of this insecticide on cotton plantations has led to the emergence of DDT resistance in A. Maculipennis. Tests against malathion and dieldrin showed that the discriminating concentration that killed 100% of the mosquitoes tested was 3.2% and 0.8% respectively.