A behavioral mutant of Paramecium tetraurelia (baA) has been isolated that has an abnormal response when placed in solutions containing Ba2+. This mutant is shown here to have a dramatic alteration of the sphingolipid and phosphonolipid composition of its ciliary membrane. This biochemical defect is present in independently isolated alleles at baA locus and segregates in crosses with the behavioral phenotype. Electrophysiologically, the mutation reduces significantly conductance of both voltage-sensitive Ca2+ channels and voltage-sensitive K+ channels. When the mutant is grown in sterol-supplemented medium, its behavior, electrophysiological properties, and lipid composition are hardly distinguishable from wild type grown under similar conditions. This mutant then, provides strong evidence that membrane lipids significantly influence the function of the membrane molecules responsible for the generation of action potentials.