Abstract Experiments are described in which a population of flies was given an acute dose of 1500 rad of X-rays every generation and then allowed to compete with a genetically similar but unirradiated population. The accumulation of second chromosome recessive lethal mutations was measured to provide an estimate of the total genetic damage and it was found that over 35% of the chromosomes carried these by generation twelve. The rate of accumulation was found to agree fairly well with a straightforward theoretical prediction. The competitive ability of the irradiated population as measured by the number of flies emerging appeared to be well maintained and was, by generation twelve, only slightly inferior to its unirradiated counterpart. The decline in the number emerging from generation one to twelve could largely be accounted for by the genetic deaths due to homozygous recessive lethals. Flies heterozygous for irradiated chromosomes, however, appeared to have a reduced competitive ability. The significance of these different responses to genetic damage in terms of the structure of the gene pool is discussed.