To produce a sex separation system for use in genetic control of Anopheles gambiae species. A, translocations were selected which linked the gene for resistance to dieldrin to the Y chromosome. Thus it could be arranged that females could be killed with dieldrin in the first instar, but males survived. In most of the stocks there were 10-15% of recombinant surviving females but in one stock (R70) the recombinant fraction was only about 0.25% maintenance of R70 by inbreeding the stock was compared with the more laborious, but also more reliable, method of outcrossing to susceptible homozygote females. Radiation sterilized R70 males were found to be fully competitive for mating in a large laboratory cage.