Irradiation of male Anopheles arabiensis Patton, followed by crossing and selection, produced a stock in which both malathion and dieldrin resistance were linked closely to the male sex. Testis squashes revealed that a translocation had been induced between one of the autosomes and the Y chromosome. Translocations of this kind could be used to allow elimination of females from batches of males being prepared for a genetic control program. However, of the two resistances which have been translocated only dieldrin could in practice be used for sex separation, because this is expressed in the larval as well as the adult stage, whereas malathion resistance was found only in the adult stage. An attempt to produce a similar sex separation system for A. stephensi Liston was unsuccessful despite the screening of 712 progeny of irradiated males.