Rab11a small G protein (Rab11p) is implicated in vesicle trafficking, especially vesicle recycling. We have previously isolated a downstream effector of Rab11p, named rabphilin-11. We found here that rabphilin-11 directly bound the mammalian counterpart of yeast Sec13 protein (mSec13p) in cell-free and intact cell systems. Yeast Sec13p is involved as a component of coat proteins II in the Sar1p-induced vesicle formation from the endoplasmic reticulum, but the precise role of mSec13p is unknown. The interaction of rabphilin-11 with mSec13p was enhanced by GTP-Rab11p. Rabphilin-11 localized on the vesicles in perinuclear regions and along microtubules oriented toward the plasma membrane, whereas mSec13p partly colocalized with rabphilin-11 in the perinuclear regions, most presumably the Golgi complex. Disruption of the rabphilin-11-mSec13p interaction by overexpression of the mSec13p-binding region of rabphilin-11 impaired vesicle trafficking. These results indicate that the rabphilin-11-mSec13p interaction is implicated in vesicle trafficking.