Within the last several years, methods have been developed for detection and analysis of tandem duplications of genetic material in bacteria and phage. Although the mechanisms for formation of these duplications are not yet completely clear, it seems reasonable to review the accumulating literature on this phenomenon. The nature and frequency of duplications raises the possibility that this mode of gene amplification could play a significant role in bacterial adaptation. It has also been suggested that gene duplication plays an important role in evolution (47, 93, 137). The high frequency of duplications helps to explain a variety of past genetic observa tions involving unstable mutants and recombinants that revert to their original state