1. 1. Prior to infection the phosphorus which eventually is incorporated into T4r bacteriophages is assimilated from the medium at the rate of bacterial growth. Before being used for phage synthesis this phosphorus must therefore have been part of general bacterial constituents like the nucleic acids. 2. 2. Phosphorus assimilated before infection appears in all the phages of the progeny but predominantly in the early finished particles. 3. 3. Within one to two minutes after infection the rate of assimilation of phage phosphorus increases by a factor of about eight. Synthesis of phage material thus seems to start very shortly after infection. 4. 4. The average time spent by phosphorus atoms between assimilation and incorporation into infective phage particles is about fourteen minutes. Early finished phages contain a mixture of phosphorus assimilated just after infection and relatively late assimilated phosphorus. The time elapsing between assimilation and incorporation of phosphorus atoms thus varies considerably. 5. 5. Bacteria infected with T4r phages probably continue to produce phage material at an undiminished rate until at least twenty minutes after infection. Some of this material will not have time enough to be incorporated into finished phage particles before lysis.