Transcripts encoding ATP synthase subunit 6 (ATP6) in petunia mitochondria were shown to be edited at 15 sites, leading to 14 amino acid changes. Certain sites are partially edited, including a site that introduces a new translation termination codon that is 13 codons upstream of the genomically encoded stop codon. Transcripts lacking the new stop codon are present in an approximately 2.5:1 ratio to transcripts carrying the stop codon created by RNA editing. To investigate whether partially edited transcripts are represented as proteins, we generated an antibody against a 12-residue peptide that is specific for translation products of unedited transcripts. This antibody did not recognize any ATP6 protein in either total mitochondrial protein preparations or ATP6 samples purified by organic solvent extraction and reverse phase HPLC procedures. According to analysis by mass spectrometry, only one form of ATP6 protein accumulates in mitochondria despite the presence of abundant partially edited transcripts. Partially edited atp6 transcripts were associated with ribosomes, suggesting that a screening mechanism(s) acts cotranslationally or post-translationally to exclude the expression of incompletely edited transcripts.