A protein was extracted from isolated nuclei of the slime mold Physarum polycephalum which could be labeled with radioactive precursors only during G(2) phase. The native protein was purified by extraction in low-ionic-strength buffer [10 mm tris(hydroxymethyl)aminomethane-hydrochloride] of isolated nuclei and by preparative polyacrylamide gel electrophoresis. It was extracted from isolated nucleoli. Its electrophoretic properties in three different polyacrylamide gel systems, its molecular weight (44,000 +/- 3,000), its precipitability by vincaleucoblastine, a vinca alkaloid, and its aggregation properties suggested that it might be actin. In a direct comparison with slime mold actin purified from the cytoplasm, no difference could be found between the two proteins in all these characteristics. The synthesis of cytoplasmic actin was not found to occur exclusively during G(2) phase. This suggested that nuclear actin was either synthesized independently from cytoplasmic actin or transported into the nuclei exclusively during G(2) phase. The possible role of nuclear actin during intranuclear mitosis is discussed.