Abstract Physico-chemical properties of Tetrahymena mitochondrial and cytoplasmic ribosomes and their components, rRNA † † Abbreviation used: rRNA, ribosomal RNA, either mitochondrial or cytoplasmic. and proteins were investigated. The mitochondrial and cytoplasmic ribosomes were 80 s and indistinguishable in sedimentation analysis at 10 −2 m-Mg 2+. However, these ribosomes exhibit different sensitivity to Mg 2+ concentrations. The cytoplasmic ribosomes consist of 60 and 40 s subunits, while the mitochondrial ribosomes produced only one sedimenting species 55 s in low Mg 2+- or EDTA-containing buffer. Buoyant densities of mitochondrial and cytoplasmic 80 s ribosomes and respective subunits after formaldehyde fixation showed reproducible differences. Mitochondrial 80 s showed a density 1.46 g/cm 3, and two density species, I = 1.52 g/cm 3 and II = 1.46 g/cm 3 were obtained in a ratio of 1:2 in amount from the 55 s subunit, while the cytoplasmic 80 s ribosomes had a density 1.56 g/cm 3 and their subunits 60 and 40 s had densities 1.57 and 1.53 g/cm 3, respectively. Electrophoretic differences exist between proteins isolated from mitochondrial and cytoplasmic 80 s ribosomes in polyacrylamide gel columns. Mitochondrial and cytoplasmic rRNA's isolated from their respective 80 s ribosomes were 21 and 14 s, and 26 and 17 s, respectively. 5 and 4 s RNA's were also present. Base compositions of mitochondrial and cytoplasmic rRNA's were different. Furthermore, the 21 and 14 s mitochondrial rRNA's were complementary to 3.8 and 1.9% of the mitochondrial DNA, respectively. No significant complementarity exists between mitochondrial DNA and cytoplasmic rRNA's. These results established that Tetrahymena mitochondria contain unique ribosomes separate from the cytoplasmic ribosomes.