In many pathogenic bacteria, genes that encode virulence factors are located in the genomes of prophages. Clearly bacteriophages are important vectors for disseminating virulence genes, but, in addition, do phage regulatory circuits contribute to expression of these genes? Phages of the lambda family that have genes encoding Shiga toxin are found in certain pathogenic Escherichia coli (known as Shiga toxin producing E. coli) and the filamentous phage CTXphi, that carries genes encoding cholera toxin (CTX), is found in Vibrio cholerae. Both the lambda and CTXphi phages have repressor systems that maintain their respective prophages in a quiescent state, and in both types of prophages this repressed state is abolished when the host cell SOS response is activated. In the lambda type of prophages, only binding of the phage-encoded repressor is involved in repression and this repressor ultimately controls Shiga toxin production and/or release. In the CTXphi prophage, binding of LexA, the bacterial regulator of SOS, in addition to binding of the repressor is involved in repression; the repressor has only limited control over CTX production and has no influence on its release.