Tetrahymena 14-nm filament protein has been shown to have dual functions as a citrate synthase in mitochondria and as a cytoskeletal protein in oral morphogenesis and pronuclear behavior during conjugation. Immunofluorescence studies of the 14-nm filament protein/citrate synthase in mitochondria found that intense mitochondrial fluorescence remained unchanged in Tetrahymena cells taken from logarithmic growth phase to stationary phase. However, electron microscopic studies showed that electron-dense rod-shaped structures found in mitochondrial matrices tended to increase in Tetrahymena cells in the growth decline phase. The rods were composed of side-by-side straight filaments with diameters of approximately 14 to 16 nm. Serial sections revealed that in Tetrahymena cells in growth decline phase, one to four electron-dense rods existed in the matrices of every mitochondrion. Immunoelectron microscopy using an anti-14-nm filament antibody clearly showed that a filament bundle of the electron-dense rod was the bundle of polymerized filaments of 14-nm filament protein/citrate synthase. These results strongly suggest that dynamic monomer-polymer conversion of the 14-nm filament protein/citrate synthase in mitochondria depends upon the physiological conditions of Tetrahymena cells.