Abstract Antibodies were raised against cytoplasmic actomyosin from Physarum polycephalum. These antibodies are directed against the myosin moiety as seen in double diffusion tests with myosin purified from the actomyosin complex. They also react with nuclear myosin, demonstrating close serological relationship of nuclear and cytoplasmic myosin in Physarum. FITC-conjugated antibodies bind to isolated nuclei of Physarum. The center of fluorescence is located in the interphase nucleolus. The same antibodies bind also to isolated macronuclei of the ciliates Ichthyophthirius multifiliis and Paramecium bursaria, indicating a serological relationship of nuclear myosins of these lower eukaryotes to Physarum cytoplasmic and nuclear myosin.