In this communication we describe the events occurring during the injection step in the infection of Escherichia coli Hfr cells with the RNA bacteriophage f2. The phage RNA is partially injected into the F-pilus of the Hfr cell. This step was found to be necessary for f2 RNA to block the penetration of the DNA phage f1. A cold sensitive mutant of f2 is described. It is able to inject its RNA at 41° but not at 31°.