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"Pawns" are mutants of Paramecium aurelia in which the process of calcium activation during membrane excitation is genetically impaired, with a corresponding loss of avoiding reactions. Mutants are selected that behave normally when grown at 23 degrees C but as pawns at 35 degrees C. The normal excitation can now be disrupted and restored in the same strain at will.

"Pawn," a genic mutant of Paramecium aurelia, cannot swim backward as the wild type can upon proper stimulation. In contrast, after membrane disruption by Triton X-100, the adenosine triphosphate-magnesium reactivated models of Pawns swim backward in the presence of calcium as wild-type models do. Thus, the mutant phenotype is due to an impairment in the membrane and not in the calcium-sensitive motile system.

Heat-sensitive Pawn (ts Pawns) of Paramecium aurelia behaved normally when grown at 23 degrees but failed to avoid strong stimuli at 35 degrees . Four of the five ts Pawn lines tested were found to be allelic at a locus known also to carry temperature-independent Pawn mutations. The fifth ts Pawn line complemented all the conditional and unconditional Pawn mutants of the two known loci. This result, together with the patterns of F(2) segregation from various crosses, suggested the existence of a third Pawn locus. An additive effect of the unlinked ts Pawn genes was observed. These findings and the significance of ts Pawns as experimental material in behavioral and physiological research are discussed.