Expression of virulence factors and motility of VIBRIO: cholerae are intimately linked by an as yet uncharacterized mechanism. Several lines of evidence indicate that the activity of the flagellum of V. cholerae might have an impact on virulence gene regulation, as alterations of the motility phenotype, either by mutation or by inhibitory drugs, result in varied levels of virulence factor production. The Na(+)-driven polar flagella of some VIBRIO: species are proposed to act as mechanosensors, sensing media viscosity. It has been suggested that the V. cholerae flagellum might act as a 'voltmeter', responding to changes in membrane potential, or might sense some environmental conditions that lead to the repression of virulence factors in V. cholerae. To test these hypotheses, beta-galactosidase levels of several types of non-motile mutant derivatives of a V. cholerae toxT::lacZ reporter strain were analysed following changes in media viscosity, membrane potential and other environmental conditions. Like the parental strain, the non-motile strain showed increased toxT::lacZ expression in high-viscosity media, suggesting that the sensing of media viscosity does not occur via the flagella. Other molecules that might be able to detect changes in media viscosity could include mechanosensitive (MS) ion channels found in the bacterial membrane. However, a V. cholerae derivative strain mutated in two putative MS channels still showed increased toxT::lacZ expression in high-viscosity media, indicating that these putative ion channels of V. cholerae are not involved in the viscosity effect and suggesting an as yet uncharacterized mechanism for sensing of media viscosity. The flagellum does not appear to act as a voltmeter, as beta-galactosidase activities of the non-flagellate derivative strain were found to be similar to those of the parental strain after artificially changing the sodium membrane bioenergetics. Similarly, several environmental conditions known to reduce toxin expression were equally effective in reducing toxT::lacZ expression in the motile or non-motile strains. In conclusion, the flagellum of V. cholerae does not act as a mechanosensor, voltmeter or signal transducer for environmental conditions. Thus, alternative mechanisms for the detection of these conditions must exist that likely do not involve the ToxR molecule, as the sensing of all of the tested parameters occurred when the TcpP/H proteins alone activated the toxT::lacZ reporter gene.