Rab11, a low molecular weight GTP-binding protein, has been shown to play a key role in a variety of cellular processes, including endosomal recycling, phagocytosis, and transport of secretory proteins from the trans-Golgi network. In this study we have described a novel Rab11 effector, EF-hands-containing Rab11-interacting protein (Eferin). In addition, we have identified a 20-amino acid domain that is present at the C terminus of Eferin and other Rab11/25-interacting proteins, such as Rip11 and nRip11. Using biochemical techniques we have demonstrated that this domain is necessary and sufficient for Rab11 binding in vitro and that it is required for localization of Rab11 effector proteins in vivo. The data suggest that various Rab effectors compete with each other for binding to Rab11/25 possibly accounting for the diversity of Rab11 functions.