Bacteria employ a variety of mechanisms to promote and control colonization of their respective hosts, including restricting the expression of genes necessary for colonization to distinct situations (i.e. encounter with a prospective host). In the symbiosis between the marine bacterium Vibrio fischeri and its host squid, Euprymna scolopes, colonization proceeds via a transient biofilm formed by the bacterium. The production of this bacterial biofilm depends on a complex regulatory network that controls transcription of the symbiosis polysaccharide (syp) gene locus. In addition to this transcriptional control, biofilm formation is regulated by two proteins, SypA and SypE, which may function in an unusual regulatory mechanism known as partner switching. Best characterized in Bacillus subtilis and other Gram-positive bacteria, partner switching is a signalling mechanism that provides dynamic regulatory control over bacterial gene expression. The involvement of putative partner-switching components within V. fischeri suggests that tight regulatory control over biofilm formation may be important for the lifestyle of this organism.