NDIVIDUAL and racial variations in the gene arrangement of chromoI somes occur in many animal and plant species. Among the sexually reproducing and cross fertilizing organisms most of the variations are caused by inversion of sections of chromosomes. Other classes of chromosomal aberrations, such as translocations, duplications, and deficiencies, occur much less frequently than the inversions, doubtless because the former are eliminated by natural selection owing to the unfavorable effects on the viability and fertility which most of them produce. The technique of the detection of inversions varies from organism to organism. Where, as in many Diptera, the giant chromosomes in the salivary gland cells are available for study, the inversions can be identified by observing the loop-like configurations formed in heterozygotes, or by examining the pattern of the stainable discs in inversions homozygotes. Inversion heterozygotes can also be identified a t meiosis through the formation of the chromatid bridges and acentric fragments in cells in which crossing over takes place within the inverted sections. The genetic method of the detection of inversion through the apparent suppression of crossing over in inversions heterozygotes is applicable only to species in which groups of linked genes are known. Flies belonging to the genus Drosophila have been more thoroughly studied with regard to inversions than any other group of organisms. These studies disclosed the remarkable fact that the characteristics of the chromosomal variability vary from species to species. Natural populations of some species appear to be uniform with respect to the gene arrangement in all chromosomes. Conversely, in other species the gene arrangement is so variable that a majority of individuals encountered in natural habitats are inversions heterozygotes. In some species the inversions occur mainly or exclusively in a single chromosome, while in others several chromosomes are subject to inversions. Observations on the variation in the gene arrangement in the chromosomes of Drosophila nebulosa Sturtevant are reported in the present article. The remarkable feature of this species is that eight out of the nine inversions so far found in natural populations are concentrated in a single chromosome, which happens to be the shortest in the chromosome set.