Enteropathogenic Escherichia coli uses a type III secretion apparatus to deliver proteins essential for pathogenesis to the host epithelium. Several proteins have been detected in culture supernatants of the prototype EPEC strain E2348/69 and three of these, EspA, EspB, and EspD, use type III machinery for export. Here, we report the identification and characterization of CesD, a protein required for proper EspB and EspD secretion. CesD shows sequence homology to chaperone proteins from other type III secretion pathways. Based on this, we hypothesize that CesD may function as a secretion chaperone in EPEC. A mutation in cesD abolished EspD secretion into culture supernatants and reduced the amount of secreted EspB, but had little effect on the amount of secreted EspA. The mutant strain was negative for both FAS and Tir phosphorylation, consistent with the previously described roles for EspB and EspD in EPEC pathogenesis. CesD was shown to interact with EspD but not EspB or EspA. CesD was detected in the bacterial cytosol, and, surprisingly, a substantial amount of the protein was also found to be associated with the inner membrane. Thus, although CesD has some attributes that are similar to other type III secretion chaperones, its membrane localization separates it from previously described members of this family.