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A class of mutants of Paramecium tetraurelia has been isolated by adding a critical amount of K + to mutagenized, autogamized cultures. Adding 35 mM K + to the medium kills the wild type. The mutants can grow at 35 mM K + and some survive an addition of up to 80 mM K + . Thirty-three lines of such mutants have been studied. These mutants have few or no behavioral abnormalities in culture medium. Paramecia can “adapt.” Hildebrand and Dryl showed that P. caudatum incubated in 15 mM K + for over 15 min fail to give avoiding reactions when challenged with Ba 2+ or a thermal gradient [Hildebrand, E. & Dryl, S. (1976) Bioelectrochem. Bioenerg . 3, 543-544]. We have observed a similar phenomenon in wild-type P. tetraurelia . However, the K + -resistant mutants show little or no such “adaptation,” i.e., they perform avoiding reactions when challenged with Ba 2+ after prolonged incubation in 15 mM K + . The K + resistance and the lack of “adaptation” are strictly correlated. Because all 33 lines of mutants show this correlation, “adaptation” and killing by K + must share a mechanism. This mechanism may be studied by finding the basis of the K + resistance in the mutants. The possible ionic bases of “adaptation” are discussed.