Under the influence of a high mutation rate gene previously extracted from a natural population, a sample of 13 independent mutations was obtained which showed a change in the sequence of genes on the X-chromosome of Drosophila melanogaster. A study of the salivary gland chromosomes revealed the presence of 12 chromosomal inversions and one transposition. The distribution of breaks along the chromosome was non-random. In all but one case, one of the breaks occurred in the proximal heterochromatic region. A study of the amount of crossingover in adjacent regions suggests that the breaks in heterochromatin are in several different locations. A correlation of crossover types in the offspring of the transposition strain with the cytological configuration and expected types revealed that about 13% of the offspring carried a chromosome with a duplication of about one-tenth of the genes. The importance of this is discussed from an evolutionary viewpoint. The results are interpreted as indicating that in addition to causing gene mutations, mutator genes are also capable of altering the sequence of genes on the chromosome by causing inversions, the nature of which depends probably upon the position of heterochromatin; and of supplying natural populations with genetic material from which genes with fundamentally new effects can evolve without the loss of the original genes.