Spirochaeta aurantia M1 cells were grown in a chemostat under conditions of energy and carbon source limitation. The chemotactic responses of the chemostat-grown cells were compared with those of S. aurantia cells grown in batch culture in the presence of excess energy and carbon source. Chemotactic responses were measured by determining the number of cells that entered a capillary tube containing a solution of attractant. S. aurantia cells grown in the chemostat under energy and carbon source limitation exhibited enhanced chemotactic responses and detected lower concentrations of attractant, as compared with cells grown in batch culture. The chemotactic response toward an attractant was specifically enhanced when that attractant was the growth-limiting energy and carbon source. The medium used contained either D-glucose or D-xylose as the sole energy and carbon source. Cells had the greatest chemotactic response toward glucose when grown at a dilution rate (D) of 0.045 h-1 under glucose limitation and toward xylose when grown at D = 0.06 h-1 under xylose limitation. When cells were grown under glucose limitation (D = 0.045 h-1), they sensed concentrations of attractant (glucose) ca. 1,000 times lower than those sensed by batch-grown cells. A similar enhancement of sensing ability (toward xylose) was observed in cells grown under xylose limitation. The results indicated that S. aurantia cells are able to regulate their chemosensory system in response to nutrient limitation. Maximum enhancement of chemotaxis occurs in cells growing at very low concentrations of energy and carbon source. Most likely, this property provides the spirochetes with competitive advantages when the availability of nutrients becomes severely limited in their habitats.