Discrete RNA fractions sedimenting slightly slower than 18s ribosomal RNA have been found in mitochondrial preparations from both hamster (BHK-21) and mouse (L-929) cells . This RNA could be separated into two components, present in approximately equimolar amounts, by prolonged zonal centrifugation or acrylamide gel electrophoresis . The hamster components had sedimentation constants averaging 16 .8 and 13 .4, and molecular weights (estimated by gel electrophoresis) averaging 0.74 and 0.42 X 106 daltons . Mixed labeling experiments showed that the mouse components sedimented and electrophoresed 3-6% more slowly than the corresponding hamster components . The RNA from both cell lines resembled mitochondrial ribosomal RNA from yeast and Neurospora in being GC poor, and in addition the larger and smaller components resembled each other in base composition . These results, taken with those of other recent studies, are compatible with the idea that our high molecular weight mitochondrial RNA is ribosomal ; such RNA would then constitute a uniquely small size-class of ribosomal RNA .