Isolated yeast mitochondria incorporate added UTP into RNA. Amongst the products formed are the two rRNAs, 4S RNA and several components presumed to be mRNAs. In omega+ strains (containing an intervening sequence in the 21S rRNA gene) besides mature 21S rRNA a transcript could be detected still containing nucleotide sequences transcribed from this intervening sequence. In omega- strains (not containing this intervening sequence) also a longer form of the 21S rRNA could be observed. These results suggest that isolated yeast mitochondria are capable of carrying out RNA synthesis and processing, including splicing.