The kinetics of host RNA synthesis, as measured by pulse-label kinetics and RNA-DNA hybridization, is inhibited 10-fold shortly after infection with bacteriophage P22. This early inhibition lasts through the first 6 min of infection and affects not only RNA synthesis but several other energy-requiring cellular functions. In lysogenic infections, the rate of bacterial transcription rapidly recovers to the value of uninfected controls. In lytic infections, the rate of host transcription increases only to about 20 to 25% of the original level, indicating a second mechanism for the inhibition of RNA synthesis in the lytic response. The early inhibition is multiplicity dependent, bhloramphenicol insensitive, and independent of phage gene 24. The lytic inhibition is dependent upon the expression of gene 24 but independent of gene 23.