The feasibility of reversing or delaying the evolution of insecticide resistance in mosquitoes by releasing susceptible males into a resistant population was demonstrated in Anopheles gamblae sensu stricto under laboratory cage conditions. Such releases produced a great reduction in the DDT resistance level of an initially resistant population. A control without releases showed a much slower decline in resistance. When DDT selection was applied to the females of the resistant population, continued male releases were able to hold the susceptibility level of t.he population constant. The males for these release experiments carried a genetic sexing system based on translocation of an insecticide resistance gene on to the Y chromosome. This allows production of batches of over 99% males for release. This system was found to be reliable and stable. The use of this translocation system was also shown to introduce semisterility into the population. In three DDT selected strains, the resistance was made homozygous and this allowed the degree of dominance to be tested after crossing to a susceptible strain. There is no DDT dose at which one can fully discriminate homozygotes and heterozygotes. The dieldrin resistance genes also carried by these three strains varied from semidominant to almost completely dominant. An experiment to test for linkage of DDT and dieldrin resistance gave conflicting results. Studies on Plasmodium infection showed that there was a tendency for loss of P. yoell nlgeriensis infection in older mosquitoes and the parasite appeared to damage young mosquitoes. A series of experiments gave no evidence of any effect of sublethal doses of DDT, malathiovi or Dimilin on susceptibility for P. y. nigeriensis or on the rate of development in refractory and susceptible strains of An. gambiae s.s and a susceptible strain of An. stenhensi. A selection experiment showed no correlation of DDT resistance to P. y. nigeriensis infection. However, DDT selection pressure produced slight cross - resistance to permethrin and higher permethrin resistance was obtained by selection with the latter compound.