Mitochondria isolated from the plasmodia of Physarum polycephalum Schw. are reacted with rabbit anti-actin antibody, and detected with FITC-conjugated sheep anti-rabbit IgG antibody. The results of indirect immunofluorescence show that actin exists in the mitochondria. Western blot analysis confirms the existence of actin in the protein preparation of the mitochondria. The indirect immunoelectron microscopic observation using the same antibodies verifies further that actin is the constituents of mitochondria, and it is dispersively distributed in the mitochondria of P. polycephalum.