Bacteria have developed mechanisms to communicate and compete with each other for limited environmental resources. We found that certain Escherichia coli, including uropathogenic strains, contained a bacterial growth-inhibition system that uses direct cell-to-cell contact. Inhibition was conditional, dependent upon the growth state of the inhibitory cell and the pili expression state of the target cell. Both a large cell-surface protein designated Contact-dependent inhibitor A (CdiA) and two-partner secretion family member CdiB were required for growth inhibition. The CdiAB system may function to regulate the growth of specific cells within a differentiated bacterial population.