The genome of a Mu-like bacteriophage D3112 specific for Pseudomonas aeruginosa was integrated in vivo into the RP4 plasmid. The fact of integration has been proved by two experiments: 1. The loss of RP4 plasmid is accompanied by loss of D3112 prophage; 2. Transfer of the plasmid by conjugation from Pseudomonas aeruginosa into bacteria of other species - P. putida PgG1 or Escherichia coli C600 leads to the occurrence of clones of these species which liberate phage capable of growing on the lawn of P. aeruginosa bacteria. The integrated state of D3112 inserted into RP4 is stable in P. aeruginosa, E. coli, P. putida. The transfer frequency of RP4 with integrated D3112 prophage into different bacteria which do not contain homoimmune prophage is essentially lower than that of the RP4 having no D3112 prophage. Specific manifestation of D3112 genome activity in E. coli cells is the sensitivity of cell growth to lower temperature (30 degrees C) of incubation.