RNA editing has been described for organellar genes of protozoa and plants as well as in three cases for nuclear transcripts of mammals. The mitochondrial tRNA for aspartic acid is encoded in the mitochondrial genome of marsupials with the anticodon GCC. However, cDNA from the gene product revealed that the second nucleotide of the anticodon of the tRNA is posttranscriptionally changed to be recognized as an uridine residue. This restores the coding property of the tRNA. Direct sequencing as well as clones of the amplified cDNA indicated that about 50% of the cellular pool of pre-tRNA molecules for aspartic acid is edited. Thus, RNA editing occurs in mammalian mitochondria and can affect the anticodons of tRNAs.