Ciliates are a good model for studying the role of actin in the nuclear spatial organization and transcription activity due to their nuclear dualism. Actin localization in the nuclear apparatus of the ciliate Paramecium caudatum was studied using fluorescently labeled phalloidin and indirect immunocytochemistry. Fibrillar actin was visualized in both nuclei. Actin was revealed in areas of the chromatin that are often associated with the periphery of the amplified nucleoli in the macronucleus. Actin distribution depended on the cell physiological state. The infection of the macronucleus with the intranuclear endobiont Holospora obtusaresulted in a loss of nuclear actin, nuclear fragility, and redistribution of the phosphorylated form of the actin binding protein cofilin. A spherical body that resembles the karyosphere was found in the macronuclear anlagen.