The genes encoding cholera toxin, one of the principal virulence factors of the diarrhoeal pathogen Vibrio cholerae, are part of the genome of CTXφ, a filamentous bacteriophage. Thus, CTXφ has played a critical role in the evolution of the pathogenicity of V. cholerae. Unlike the well-studied F pilus-specific filamentous coliphages, CTXφ integrates site-specifically into its host chromosome and forms stable lysogens. Here we focus on the CTXφ life cycle and, in particular, on recent studies of the mechanism of CTXφ integration and the factors that govern lysogeny. These and other processes illustrate the remarkable dependence of CTXφ on host-encoded factors.