Many bacterial pathogens regulate the expression of virulence genes in a co-ordinate manner in response to changes in the environment. For example, the human pathogen, Vibrio cholerae, possesses a virulence regulon composed of over 20 genes involved in colonization, toxin production and bacterial survival within the host, which are co-ordinately regulated by external stimuli, such as temperature, pH and osmolarity. Although the expression of the regulon is dependent upon the transcriptional activator ToxR, most of these genes are controlled by a second transcriptional activator, ToxT, which is itself positively regulated by ToxR. The mechanisms by which environmental stimuli influence the ToxR regulon are not yet understood, but ToxR-mediated control over the expression of toxT clearly plays a role. The recent finding that the global regulator cAMP-CRP also influences the expression of the ToxR regulon under various environmental conditions raises new issues regarding the pathways and mechanisms by which this regulation is achieved and indicates that multiple overlapping systems are involved.