Gene expression in a small phage (∅29) and a large phage (∅e) was followed during differentiation of the host, Bacillus subtilis 168. It was found that ∅29 DNA replication becomes inhibited relatively early in the sporulation cycle; though unable to replicate, ∅29 DNA can be incorporated into spores in a heat-stable form and expressed later after germination. Considerable amounts of “early” ∅29-specific mRNA appear in infected cells even at times during cell development when very little ∅29 DNA syntehsis occurs. ∅e DNA, on the other hand, can replicate during later stages of sporulation. The synthesis of ∅e-specific mRNA is observed in cells infected more than an hour after the time of inhibition of DNA synthesis. These results show that transcription of phage DNA occurs in vivo well after the occurrence of host changes that inhibit phage replication. Therefore, the changes in the template specificity of host RNA polymerase probably cannot account for the inability of phage to multiply at early stages of sporulation, as has been previously suggested from in vitro studies.