Two programmes of repeated backcrossing to a susceptible triple-mutant marker strain and a susceptible unmarked strain with selection for certain mutant phenotypes and with DDT, plus a third programme of repeated back crossing to the susceptible unmarked strain with permethrin selection were undertaken in an attempt to isolate the DDT-resistance genes, RDDT and RDDT2, and the pyrethroid-resistance gene, Rpy. The three selected lines were then inbred and further selected with DDT or permethrin to make the isolated genes homozygous. The accumulated data from tests at various stages with permethrin, DDT and DDT plus the synergist FDMC, a blocker of dehydrochlorination, produced an apparently simple picture of the relationship between DDT and pyrethroid resistance in adult Aedes aegypti. Two major DDT resistance genes can be present; one, RDDT, located on chromosome II, controls the resistance mechanism dehydrochlorination and confers a level of DDT resistance 3-4 x higher than the other, but produces no cross-resistance to permethrin. RDDT2, on chromosome III, is allelic to Rpy; when isolated in a susceptible background it confers resistance to DDT of about 10-14 x and cross-resistance to permethrin of 18-21 x.