Populations of Drosophila melanogaster were maintained for 36 generations in r- and K-selected environments in order to test the life-history predictions of theories on density-dependent selection. In the r-selection environment, populations were reduced to low densities by density-independent adult mortality, whereas populations in the K-selection environment were maintained at their carrying capacity. Some of the experimental results support the predictions or r- and K-selection theory; relative to the r-selected populations, the K-selected populations evolved an increased larval-to-adult viability, larger body size, and longer development time at high larval densities. Mueller and Ayala (1981) found that K-selected populations also have a higher rate of population growth at high densities. Other predictions of the thoery are contradicted by the lack of differences between the r and K populations in adult longevity and fecundity and a slower rate of development for r-selected individuals at low densities. The differences between selected populations in larval survivorship, larval-to-adult development time, and adult body size are strongly dependent on larval density, and there is a significant interaction between populations and larval density for each trait. This manifests an inadequacy of the theory on r- and K-selection, which does not take into account such interactions between genotypes and environments. We describe mechanisms that may explain the evolution of preadult life-history traits in our experiment and discuss the need for changes in theories of density-dependent selection.