Abstract
The P2 phage mutation vir56, like the previously studied vir22, is the result of an unequal replacement of a chromosome segment with non- homologous DNA. The end positions of the replacements are essentially the same in the two mutants, whereas the lengths of the replacements are quite different. A third chromosomal aberration, del3, has similar structure and position. These results strengthen the suggestion that the left ends of these three aberrations coincide with the point of exchange in integrative recombination.