Bacillus subtilis W23 was infected with bacteriophage SP-15. Two waves of phage-specific RNA synthesis were observed. Wave I was prereplicative, and wave II was coincident with replication of the viral genome. To determine the temporal appearance of general classes of phage-coded messengers and proteins, we studied the dependence of lysozyme synthesis, phage production, and DNA synthesis on time of addition of transcriptional and translational inhibitors. Lysozyme synthesis started to become refractile to a variety of transcriptional inhibitors (rifampin, streptolydigin, and actinomycin D) between 20 and 22 min postinfection and was completely refractile by 30 min. Nevertheless, functional enzyme did not appear until 45 to 47 min postinfection; lysozyme was maximal by 65 min. Rna isolated from SP-15 phage-infected cells was used to program the cell-free synthesis of lysozyme. The messenger was synthesized exclusively between 20 and 30 min postinfection. Lysozyme messengers were stable. The data imply that lysozyme messengers were present 52 min prior to their translation. Progeny virus formation remained sensitive to transcriptional inhibitors until 40 to 50 min postinfection, and sensitivity to chloramphenicol lasted 65 min. The first progeny viruses appeared at 75 min. Again, an unusually long lag between completion of functional messengers and their translation was evident. The aforementioned data indicated that transcription of lysozyme messengers and, at least, some messengers, whose products are essential for phage production, are uniquely associated with waves I and II of RNA synthesis, respectively. However, messengers whose products are essential for normal amounts of DNA synthesis were apparently synthesized during both waves; transcription of these messengers was transiently repressed (using the term broadly) between 30 and 40 min postinfection. Judging from the dependence of DNA synthesis on time of chloramphenicol addition, proteins essential for normal amounts of DNA synthesis were also synthesized in two discrete waves, each yielding sufficient protein for half-maximal levels of DNA synthesis. An hiatus in the synthesis of the proteins in question was evident between 45 and 65 min postinfection; evidence cited in this paper indicates that this hiatus did not result from messenger depletion, which, in turn, implied some type of translational-level control. This latter conclusion is substantiated by the lysozyme synthesis that occurred during the same interval when synthesis of certain proteins for DNA replication was transiently repressed.