THE ciliated protozoan Paramecium provides an opportunity to study the electrophysiological properties of an excitable membrane in relation with simple locomotor behaviour. The Paramecium surface membrane controls the locomotor activity, in particular ciliary reversal (resulting in backward swimming), by ionic conductance mechanisms like those in other excitable cells1,2. Depolarising stimuli (electrical, chemical or mechanical) cause a graded receptor potential due to a regenerative transient increase in calcium conductance and a delayed increase of potassium conductance. The resulting Ca2+ influx causes an increase in the intracellular Ca2+ concentration which initiates the reversal of the ciliary beating direction. Normal beating is resumed when the excess internal Ca2+ has been removed by metabolism-dependent processes.