Growth of phages phi W and T7 was restricted in Escherichia coli lysogenic for phage P1. Only a fraction of the infected cells gave burst of phages. Cells permitting phage growth gave normal burst size. Host strains carrying P1 mutants with defective endonuclease gave no restriction of phages T7 and phi 3, the latter a host-range mutant of phi W. Degradation but not modification of parental phage DNA could be demonstrated. Although no DNA, RNA or protein was synthesized in phi W infected P1 lysogenic cells, the parental phage DNA was found in increasingly larger complexes during the course of infection. At early times after infection, parental phage DNA was found to sediment about twice as fast as mature phage DNA. At later times during the infection the parental phage DNA was recovered as a very rapidly sedimenting material. Such material was also found in alkaline sucrose gradient centrifugation after treatment of the cell extract with sodium dodecyl sulphate, pronase digestion and phenol extractions.