In this paper we demonstrate the presence and localization of calmodulin, a calcium-dependent regulatory protein, in the ciliated protozoan Paramecium tetraurelia. Calmodulin is demonstrated by several criteria: (a) the ability of whole cell Paramecium extracts to stimulate mammalian phosphodiesterase activity, (b) the presence of an acidic, thermostable, 17,000-dalton polypeptide whose mobility shifts in SDS polyacrylamide gel electrophoresis in the presence of Ca2+, and (c) the affinity of antibodies against mammalian calmodulin for a Paramecium component as demonstrated by both indirect immunofluorescent localization and radioimmunoassay. Indirect immunofluorescence studies reveal that Paramecium calmodulin is distributed in three distinct regions of the cell, i.e., (a) large, spherical cytoplasmic organelles representing perhaps the food vacuoles or other vacuolar inclusions of the cell, (b) along the entire length of oral and somatic cilia, and (c) along a linear punctate pattern corresponding to the kinetics (basal bodies) of the cell.