As part of a study on the suitability of translocations for insect pest control, artificial selection was applied for either higher or lower egg hatchability in each of the reciprocal matings between a translocation heterozygote and a translocation homozygote. In each of four selection lines, there was response to selection but, after 3–4 generations, limits were reached beyond which further selection gave no response. On reversing the directions of selection, the high and low lines rapidly exchanged their levels of egg hatchability and then established new plateaux. Relaxation of selection caused convergence towards the original unselected level. It is concluded that individuals with extremely high or low fertility were disfavoured by natural selection. Populations initiated from two different translocation homozygotes formed a stable polymorphism and after propagation in bottles for 10 generations, small increases were found in the fertility of the double translocation heterozygotes compared with the same genotype newly produced from unselected homozygote stocks. It is concluded that, under the conditions of the bottle cultures, natural selection favoured increase in fertility of the double heterozygotes.

A reciprocal translocation between chromosome 2 and 3, designated T2, was viable when homozygous in the ROCK strain ofAedes aegypti. It was backcrossed five times with Delhi wild type material. Despite intensive efforts it was not possible to re-isolate it as a homozygote, indicating that a factor in the Delhi background interacted with the translocation and caused recessive lethality. In certain families inbreeding without the production of genetically marked non-translocation homozygotes suggested that a translocation homozygote line had been isolated but, when outcrossed, all the individuals were found to be translocation heterozygotes. It was shown that a balanced lethal system existed which maintained permanent translocation heterozygosity in this line.

Cycling populations ofA. aegypti of wild origin were established in outdoor cages. Releases were then made for 32–43 days of either males carrying chromosome translocations or males of the sex ratio distorter type. The translocation caused a maximum of 50% sterility, but this declined rapidly after termination of releases. The distorter males depressed the proportion of females among the pupae produced in the cage to a minimum of 35% and the distortion of sex ratio persisted for 13 weeks following termination of releases. It was pos sible to simulate the effects of the releases with a computer. Simulations were also made of standard release schedules of three types of genetic material. A strain carrying both sex ratio distortion and a translocation gave the most effective population suppression.

Previously reported effects of aging and mating history in causing partial loss of cytoplasmic incompatibility in males of Paris cytoplasmic type, have been shown also to occur in males with Delhi and Hamburg cytoplasmic types. It was unexpectedly found that increasing the ratio of females to males at the first mating did not lead to enhancement of the level of partial compatibility when the males were re-mated. The effect of aging is discussed in relation to the use of cytoplasmic incompatibility integrated with chromosomal abnormalities, for genetic control.