This is a review of Paramecium electrophysiology and its relation with motility. When stimulated electrically, the membrane produces a graded calcium-based action potential, through voltage-gated channels located in the cilia. When calcium enters the cilia, the cilia reorient, which makes Paramecium swim backward for a little while, until calcium is expelled or buffered, and then cilia reorient and beat again in the forward direction. Swimming direction changes at the transition. This is called the “avoiding reaction”. This can be triggered mechanically by touching the anterior part of the cell, which opens mechanosensitive calcium channels. Touching the posterior part opens mechanosensitive potassium channels, resulting in hyperpolarization, which then triggers the “escape reaction”: Paramecium swims forward at higher velocity.