AbstractIn its normal culturing environment, the trypanosomid flagellate Crithidia oncopelti propagates basally‐directed planar waves, but may under certain conditions exhibit base‐to‐tip wave propagation in what is regarded as an avoidance response. The beat frequency and wave shape in both modes of beating are dependent on the viscosity of the swimming medium; viscosity may also influence the direction of wave propagation. If Crithidia experience a sudden increase in viscosity, there is a marked increase in the proportion of the population that is seen to exhibit wave propagation from base to tip; this proportion gradually decreases with time until the whole sample has reverted to “normal” beating. In a single organism, the resumption of normal beating is not accomplished in a single transition but by a series of switches between the forward and reverse modes. The interval of time between successive switches appears to be random, while the length of time spent in base‐to‐tip wave propagation gradually decreases. Despite the randomness of the switching process, its rate when averaged over successive time intervals is found to be constant at a particular viscosity and also dependent on it. The precise manner by which this organism is able to control its direction of wave propagation is unclear. However, the switching behavior it exhibits during the period of adaptation to an increased mechanical loading of the flagellum may reflect a process that characterizes a facet of this controlling mechanism.