The temperate phage SPβ infects the endospore-forming Gram-positive bacterium Bacillus subtilis and resides as a prophage in its genome. Phages of the SPβ group use a peptide-based communication system (termed the “arbitrium” system) to coordinate lysis-lysogeny decisions. Recently, we have identified the master repressor of SPβ and a factor that acts downstream of the repressor and is critical for the activation of the lytic cycle. Although many additional components of the lysis-lysogeny decision system have already been identified, it is unclear how the lysogenic cycle, particularly in the case of the phage SPβ, is activated. By pursuing genetic, biochemical, and structural studies, we are elucidating the molecular details of the lysis-lysogeny decision system of the temperate phage SPβ. We also study a putative SPβ defense system that is encoded on an Escherichia coli–B. subtilis shuttle vector.