The Rcs phosphorelay is an important signalling pathway that is conserved throughout the Enterobacteriaceae. The Rcs phosphorelay is composed of the RcsC sensor kinase, a membrane-localized HPt-containing protein RcsD and the cytoplasmic response regulator RcsB. In this study we were interested in studying the degree of functional conservation between the different enteric RcsC homologues. Therefore, we tested for the ability of RcsC homologues from pathogenic Escherichia coli, Salmonella enterica and Yersinia pestis to functionally complement an rcsC mutation in E. coli K-12. Complementation was measured as (1) an increase in cpsB-lacZ expression in response to DjlA overproduction, a well-established signal for RcsC activation and (2) the ability to restore biofilm formation. All of the homologues increased cpsB-lacZ expression in response to DjlA overproduction confirming that the different RcsC homologues are able to sense this stimulus and to transduce the signal to the downstream components of the Rcs pathway in E. coli. This suggests that the core signalling domains of RcsC are functionally conserved throughout the Enterobacteriaceae. However, we also show that RcsC from Y. pestis was unable to restore normal biofilm formation in the E. coli K-12 rcsC mutant and we argue that this is due to the increased net kinase activity observed with this homologue.