Virulence of Vibrio cholerae depends on secretion of cholera toxin (CT), which is encoded within the genome of a filamentous phage, CTX phi. Release of CT is mediated by the extracellular protein secretion (eps) type II secretion system. Here. the outer membrane component of this system, EpsD, was shown to be required for secretion of the phage as well. Thus, EpsD plays a role both in pathogenicity and in horizontal transfer of a key virulence gene. Genomic analysis suggests that additional filamentous phages also exploit chromosome-encoded outer membrane channels.