Mutants of Escherichia coli were selected in which a single mutational event had both relieved the polar effect of an early trpE mutation on trpB and simultaneously released the expression of trpB from tryptophan repression. The frequency at which these mutations appeared was roughly equal to the frequency of point mutations. In each of these mutants, the mutation increased the function of trpB and also increased the activity of some, but not all, of the other four tryptophan operon genes. Genetic analysis showed that the mutations were not located within the trp operon since in each case the parental trp operon could be recovered from the mutants. Each mutant was shown to carry a duplication of a trp operon segment translocated to a new position near the trp operon. Polarity is relieved since the trpB duplication-translocation is not in the same operon as the trpE polar mutation. The duplicated and translocated segments are fused to operons not regulated by tryptophan, so trpB function is no longer subject to tryptophan repression. The properties of the mutants indicate that the length of the duplicated segment and the position to which it is translocated differ in each of the seven mutants studied. The duplications are unstable, but the segregation pattern observed is not consistent with a single crossover model for segregation. That such duplication-translocation events generate a variety of new genetic arrangements at a frequency comparable with point mutations suggests they may play an important role in evolution.