Certain marine Vibrio species swim in sea water, propelled by a polar flagellum, and swarm over surfaces using numerous lateral flagella. The polar and the lateral flagellar motors are powered by sodium- and proton-motive forces, respectively. The lateral flagella are produced in media of high viscosity, and the relevant viscosity sensor is the polar flagellum. The cell might monitor either the rotation rate of the flagellar motor or the mechanical force applied against the flagellum. To test these possibilities, we examined the effects of amiloride and its derivatives, which inhibit the rotation of the sodium-driven motor, on lateral flagellar gene (laf) expression in Vibrio parahaemolyticus. Phenamil, an amiloride analogue that inhibits swimming at micromolar concentrations, induced laf transcription in media devoid of viscous agents in a dose-dependent manner. The relationship between the average swimming speed and laf induction in the presence of various concentrations of phenamil was very similar to that observed when viscosity was changed. These results indicate that marine Vibrio sense a decrease in the rotation rate of (or the sodium influx through) the polar flagellar motor as a trigger for laf induction. Alternative mechanisms for laf induction are also discussed.