The decision between lytic and lysogenic development of temperate DNA bacteriophages is determined largely by transcriptional regulation through DNA-binding proteins. To determine whether a heterologous RNA-binding activity could control the developmental fate of a DNA bacteriophage, a derivative of P22 was constructed in which the chosen developmental pathway is regulated by an RNA-binding molecule interacting with its RNA target site located in a phage mRNA. In the example presented, lysogenic development of the phage relies upon R17 coat protein expression in the susceptible host cell and the availability of a suitable coat protein binding site encoded by the phage genome. Through the analysis of phage mutants that are able to grow lytically in susceptible cells that express the coat protein, additional insights were obtained regarding the specific interaction of the R17 coat protein with its RNA binding site. This study also suggests a novel and extremely sensitive strategy for selecting RNA-binding activities in vivo.