Mitochondrial RNA from grande yeast was analyzed by electrophoresis on agarose-urea, acrylamide-urea, and methyl mercuric hydroxide-agarose gels. These gel systems display more than 40 RNA bands that copurify with mitochondria; these bands are not present in cytoplasmic RNA preparations. Analysis of molecular weight on methyl mercuric hydroxide gels indicates a size range of 200 to 9,500 nucleotides, including 11 species larger than 21S rRNA (3,700 nucleotides). The mitochondrial origin for many of these species was further verified by transfer of RNA from gels to diazobenzyl oxymethyl paper and hybridization to (32)P-labeled mitochondrial DNA. The total molecular weight of the catalogued RNA species was approximately 110,000 nucleotides, considerably greater than the size of the 76,000-base-pair genome. These results suggest that large primary transcripts are processed by multiple cleavages to mature RNA species.