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Abstract. To understand the role of calcium ions in thigmotaxis in Paramecium caudatum, the effects of caffeine, ruthenium red and lanthanum (LaC13) on thigmotaxis were examined. Thigmotaxis in the CNR mutant, which lacks voltage-dependent Caa+-channels in the ciliary membrane, was also examined. Ruthenium red and LaCI3 suppressed thigmotaxis in P. caudatum, while caffeine enhanced it. The CNR mutant showed hardly any thigmotaxis. It can be thought that an increase in Ca 2 § influx and the intraciliary concentration of Ca 2 § ions induces thigmotaxis in Paramecium.