Fundamental to all living organisms is the capacity to coordinate cell division and cell differentiation to generate appropriate numbers of specialized cells. Whereas eukaryotes use cyclins and cyclin-dependent kinases to balance division with cell fate decisions(1), equivalent regulatory systems have not been described in bacteria. Moreover, the mechanisms used by bacteria to tune division in line with developmental programs are poorly understood. Here we show that Caulobacter crescentus, a bacterium with an asymmetric division cycle, uses oscillating levels of the second messenger cyclic diguanylate (c-di-GMP) to drive its cell cycle. We demonstrate that c-diGMP directly binds to the essential cell cycle kinase CckA to inhibit kinase activity and stimulate phosphatase activity. An upshift of c-di-GMP during the G1-S transition switches CckA from the kinase to the phosphatase mode, thereby allowing replication initiation and cell cycle progression. Finally, we show that during division, c-di-GMP imposes spatial control on CckA to install the replication asymmetry of future daughter cells. These studies reveal c-di-GMP to be a cyclin-like molecule in bacteria that coordinates chromosome replication with cell morphogenesis in Caulobacter. The observation that c-di-GMP-mediated control is conserved in the plant pathogen Agrobacterium tumefaciens suggests a general mechanism through which this global regulator of bacterial virulence and persistence coordinates behaviour and cell proliferation.