The effect of assortative mating on selection response at high heritability and high selected proportion was studied in Drosophila melanogaster. Six generations of selection for increased pupa length were carried out in two assortatively mated and two randomly mated lines. No clear and consistent superiority in response was found for assortative mating, in part because the use of reserve pairs as parents generated some degree of assortative mating in the random mating lines. An analogous situation might be found in livestock populations, which are often divided in tiers of different genetic level, mating taking place within tiers. Thus, some degree of assortative mating could already being contributing to selection response in those populations, and the use of additional deliberate assortative mating in animal breeding plans may be of limited interest.