The torque-speed relationship of the Na+-driven flagellar motor of Vibrio alginolyticus was investigated. The rotation rate of the motor was measured by following the position of a bead, attached to a flagellar filament, using optical nanometry. In the presence of 50 mM NaCl, the generated torque was relatively constant (∼3800 pN nm) at lower speeds (speeds up to ∼300 Hz) and then decreased steeply, similar to the H+-driven flagellar motor of E. coli. When the external NaCl concentration was varied, the generated torque of the flagellar motor was changed over a wide range of speeds. This result could be reproduced using a simple kinetic model, which takes into consideration the association and dissociation of Na+ onto the motor. These results imply that for a complete understanding of the mechanism of flagellar rotation it is essential to consider both the electrochemical gradient and the absolute concentration of the coupling ion.