The Caulobacter crescentus chromosomal clp locus contains the genes encoding the components of ClpXP, a multisubunit protease required for cell cycle progression in this organism. Here, we report the identification and characterization of cicA, a gene located between the clpX and clpP genes on the Caulobacter chromosome. cicA is a novel morphogene in C. crescentus and, like clpX and clpP, is essential for growth. A conditional cicA mutant stopped growth, but retained viability under restrictive conditions. In contrast, an increased concentration of CicA led to an immediate loss of the normal rod shape, an almost 10-fold increase of the cell's volume and a cell division block. In parallel with this drastic morphological change, cells rapidly lost viability. Primary sequence analysis suggested that the cicA gene encodes a member of a large superfamily of phosphotransferases, that include phosphoserine phosphatases, the ATPase domain of P-type ATPases and receiver domains of response regulators. Four conserved motifs of this protein family that have been implicated in the catalysis of phosphotransfer reactions were investigated by site-directed mutagenesis and were found to be critical for in vivo function of CicA. Based on our observations, we postulate that CicA is involved in essential phosphotransferase reactions in Caulobacter and that increased activity of CicA has a deleterious effect on cell wall biosynthesis, morphogenesis and cell division.