A computer program has been designed to study behavior in populations of Spirochaeta aurantia cells, and this program has been used to analyze changes in behavior in response to chemoattractants. Three kinds of behavior were distinguished: smooth swimming, flexing, and reversals in direction of swimming after a short pause (120 ms). Cell populations exposed to chemoattractants spent, on average, 66, 33, and 1% of the time in these modes, respectively. After the addition of a chemoattractant, behavior was modified transiently--smooth swimming increased, flexing decreased, and reversals were suppressed. After addition of D-xylose (final concentration, 10 mM), the adaptation time (the time required for the populations to return to the unmodified behavior) for S. aurantia was 1.5 to 2.0 min. A model to explain the behavior of S. aurantia and the response of cells to chemoattractants is described. This model includes a coordinating mechanism for flagellar motor operation and a motor switch synchronizing device.