By entering a reversible state of reduced metabolic activity, dormant microorganisms are able to contend with suboptimal conditions that would otherwise reduce their fitness. In addition, certain types of dormancy like sporulation, can serve as a refuge from parasitic infections. Phages are unable to attach to spores, but their genomes can be entrapped in the resting structures and are able to resume infection upon host germination. Thus, dormancy has the potential to affect both the reproductive and survival components of phage fitness. Here, we characterized the distribution and diversity of sigma factors in nearly 3,500 phage genomes. Homologs of bacterial sigma factors that are responsible for directing transcription during sporulation were preferentially recovered in phages that infect spore-forming hosts. While non-essential for lytic infection, when expressed in Bacillus subtilis, we demonstrate that phage-encoded sigma factors activated sporulation gene networks and reduced spore yield. Our findings suggest that the acquisition of host-like transcriptional regulators may allow phages to manipulate the expression of complex traits, like the transitions involved in bacterial dormancy. Overall design: RNAseq of strains with sigma factors cloned under an IPTG-inducible promter in an ectopic locus (amyE). Cultures of each strain (n=3) were grown in LB to mid-exponenial phase, at which time the culture was split into two, and IPTG (1mM) was added to one half. RNAwas extracted two hours after induction. Differential expression was inferred from comparison of a the induced culture to its non-induced paired control.