Protozoan are unicellular eukaryotes that display many animal-like behaviors. Among protozoan, ciliates form a complex and diverse group that swim by beating many cilia. Many ciliates are extremely mechanosensitive. Some, like Stentor and Spirostomum, contract in a few milliseconds when they are touched. Others, like Paramecium, swim backward when touched on the anterior end, or accelerate when touched on the posterior side. This mechanosensitivity is known to be mediated by mechanoreceptors, which trigger a transmembrane ionic current when the membrane is stressed. In addition, the mechanosensitive behavior of ciliates shows habituation, a phenomenon first studied in animals, considered the most elementary form of learning. When the organism is repeatedly stimulated, its reaction weakens (for example, it stops contracting); after stopping the stimulation for a while, the organism becomes sensitive again. Habituation has been characterized most exhaustively in the contraction behavior of Stentor, but we have recently observed it in the modulation of swimming in Paramecium, which is more amenable to electrophysiogical studies (measurement of mechanotransduction currents). The goal of this project is to develop a biophysical model of mechanotransduction and habituation in ciliates.