The Res phosphorelay is composed of RcsC, RcsD and the response regulator RcsB, and this signalling pathway has been implicated in virulence and biofilm formation in many enteric bacteria. It was previously shown that a mutation in rcsC resulted in defective biofilm formation in Escherichia coli [Ferrieres, L. & Clarke, D. J. (2003) Mol Microbiol 50, 1665-1682]. To identify the molecular mechanisms underlying the observed biofilm defect we carried out a screen looking for suppressor mutants that restored biofilm formation in the rcsC mutant background. One of the mutants was identified to be in rprA, a gene encoding a small RNA molecule that is involved in the post-transcriptional control of the alternative sigma factor, sigma(s). The expression of rprA is regulated by the Res phosphorelay, and there are elevated sigma(s) levels present in the rcsC mutant due to the overexpression of rprA in this background. Using different approaches, we have established that the increase in sigma(s) levels is responsible for the biofilm defect. Therefore, the Res phosphorelay is involved in maintaining appropriate levels of sigma(s) during biofilm formation in E coli.