We have used surface plasmon resonance biosensor technology to monitor the assembly and dynamics of a signal transduction complex which controls chemotaxis in Escherichia coli. A quaternary complex formed which consisted of the response regulator CheY, the histidine protein kinase CheA, a coupling protein CheW and a membrane-bound chemoreceptor Tar. Using various experimental conditions and mutant proteins, we have shown that the complex dissociates under conditions that favour phosphorylation of CheY. Direct physical analysis of interactions among proteins in this signal transduction pathway provides evidence for a previously unrecognized binding interaction between the kinase and its substrate. This interaction may be important for enhancing substrate specificity and preventing 'crosstalk' with other systems. The approach is generally applicable to furthering our understanding of how signalling complexes transduce intracellular messages.