The pathogenicity of Vibrio cholerae depends upon its production of two key virulence factors: the toxin co-regulated pilus (TCP), a colonization factor, and cholera toxin, an exotoxin. Genes encoding both virulence factors were introduced into V. cholerae by horizontal gene transfer. The toxin genes are contained within the genome of CTXphi, an integrated filamentous phage identified in 1996. In the past few years, it has been shown that CTXphi relies on novel processes for phage DNA integration, replication and secretion. In addition, expression of CTXphi genes--including the toxin genes--and transmission of CTXphi were recently found to be promoted by the antirepressor RstC, which is encoded within RS1, a newly described satellite phage of CTXphi. The genetic island that encodes TCP has also been described as a filamentous phage; however, these sequences are unlike the genome of any previously characterized filamentous phage.