Swarmer cell genes, laf, are induced when V. parahaemolyticus is grown on the surface of solidified media, embedded in solidified media, suspended in viscous media, or agglutinated with antibody in liquid media. These conditions have in common the constraint of the movement of the polar flagellum. To test the hypothesis that the polar flagellum functions as a sensor necessary for control of swarmer cell formation, we have constructed a variety of mutations in genes encoding components of the polar flagellum, fla. The consequence of such mutations is the constitutive expression of laf genes. So, the performance of the polar flagellum is coupled to the transcription of laf genes such that when function is perturbed, either physically or genetically, swarmer cell genes are induced. Because the polar flagellum appears to be capable of sensing external forces influencing its motion, we suggest it is acting as a dynamometer.