The role of a $Na^+$ electrochemical potential on protein translocation was examined. The translocation of E. coli uncleavable OmpF-Lpp into inverted membrane vesicles prepared from $Na^+$ pump-containing V. alginolyticus was little inhibited in the presence of the proton conductor, carbonyl cyanide m-chlorophenyl-hydrazone (CCCP). In contrast, monensin, which dissipates a $Na^+$ electrochemical potential, Significantly inhibited the translocation of uncleavable OmpF-Lpp to the level of translocation activity observed in the absence of NADH. These results suggest that the electrochemical potential of $Na^+$, but not that of $H^+$, plays an important role in protein translocation in V. alginolyticus.