Two neighbouring natural populations of Drosophila melanogaster have been analysed, one from a cellar habitat and the other from a vineyard outside. An extensive study of inversion polymorphism in the two populations has been carried out. Furthermore, the relationship between inversion polymorphism and the viability of the second chromosome has been studied. The data regarding the total frequency of inversion-carrying chromosomes indicate a lower frequency in the cellar population than in the vineyard population. Some possibilities that could explain the behaviour of the chromosomes from the cellar in relation to the peculiar environment of this habitat are discussed. New endemic inversions have been detected in both populations. With respect to the fitness component studied, no differences seem to exist between the cellar and vineyard populations. The frequencies of lethal-carrying chromosomes were the same in the two populations (0.267). There were no significant differences between the distribution patterns of the two populations for homozygote or for heterozygote viabilities. Data on allelism rates of lethals and population sizes help us to characterize certain aspects of both populations.