Abstract Genetically marked P2 phage was used to superinfect cells of Escherichia coli (or Shigella dysenteriae ) which were lysogenic for P2 and hence immune to the superinfecting phage. The genetic incorporation of the superinfecting P2 was studied by examining the progeny of the superinfected cells. The results obtained are in agreement with the previously established rule that genetic incorporation occurs most frequently at the preferred location (I) either by prophage addition or by prophage substitution. Besides complete substitution, partial (= single marker) substitution was also observed, but at a lower frequency than complete substitution. The frequency of genetic incorporation was found to increase linearly with the multiplicity of superinfection for small multiplicities. The probability of genetic incorporation of a superinfecting phage is decreased by the presence of a prophage in location I; this hindrance does not depend on the immune specificity of such prophage. It is assumed that in immune cells multiplication of prophage precursors (= preprophages) is prevented. The probability of genetic incorporation of a preprophage in the absence of steric hindrance (5%) compares well with the probabilities found in other systems for the genetic incorporation of bacterial markers in transduction experiments. Weak virulent mutants of P2 and P2 Hy dis can become prophages, but only in cells already lysogenic for a temperate phage of the same immunity class.